I shoot NRA Highpower, Long Range, and now I shoot PRS and Palma stuff for fun. I was good enough at one point to make it on a few winning teams. It's worth paying attention how everyone else does it. This is a response I wrote to a similar question. I'll put new stuff that I am adding to it in italics. There are a lot of different ways to come up with your best educated guess.
Strong Wind (15mph)
I stole the idea from a data-sheet for the "British Wind" table which was for 7.62 which basically was a strong wind will move a bullet 1 MOA per 100 yards. So it was 100=1 200=2 300=3 etc. It's something I learned when I shot M14s for fun. I converted the idea to apply for Mils and 6.5mm bullets. Basically, I count by 2s from 100-300, and then by 3s out to 1000, and by .4s after that. I am also fairly certain that a couple of different professional instructors teach similar methods.
(I also really like the 6mph Method highlighted earlier here as well. That's pretty smart)
The whole purpose of the exercise isn't necessarily to give you a direct output, but more to directly visualize how much your bullet is going to be moved. The idea is to take your strong wind as your benchmark and then reduce those values to 1/2 and 1/4 for a Medium, and Light wind.
A 15 mph wind is going to be a nice day at the beach, or just fast enough to make the Georgia heat actually pleasant. Also according to the Beaufort scale, it will raise dust and paper, so it's probably also going to be the point where you start putting an ammo box on your match-book to keep things from blowing away.
A 7 mph wind is right about where leaves of trees and grass should be in constant motion. It's also the point where the gnats have gone away 20 minutes ago but you haven't noticed.
A 3 mph wind is where you can feel it, and you are wondering why the gnats are still there, and why are there even gnats in the first place. It will also move grass and smoke, but it might be difficult to determine direction.
Sorry, I was just remembering all the gnats at the last match I shot.
Wind Bitch Method
Working with other shooters is a huge advantage. Good shooters tend to group together because they know it gives them an edge. Having the equivalent of a Highmaster shooter come off the firing line and tell you what he just used for wind is a huge advantage. When I am actually trying to be good at a match, I will try to squad with good shooters because the information I get from watching good shooters shoot is immediately valuable.
If the match allows for it, I will try to watch someone shoot through a spotting scope, and then ask them what they used for wind. It helps confirm or deny the wind plan that I would have came up with if I was shooting all alone, or didn't have someone to use as a wind dummy.
Databook
I've moved away from keeping meticulous data at matches due to laziness. However, it is something that I am going to start doing again because I think that it gives you a huge edge in collecting your thoughts, preparation, and self-reflection. Writing things down on paper in a format that is easy to come back to can pay off huge dividends when it comes to breaking the learning curve. Attached is a picture of a generic UKD datasheet that we used to use. It's basically an adaptation of Highpower. It doesn't have to always be filled all the way out.
All of the stuff on top is basic information that should be familiar. This is part of our planning phase for a stage. In our target box, I wrote IPSC, but I also sometimes draw the shape of the target.
In the remarks section, I put basic info about the course of fire, and that the previous shooter used 2 mils of wind at 1200 and that it was off to the right.
There is an elevation and windage box. I write in all the data I intend to use for the stage, and then any corrections I made. In this case, my data past 900 wasn't correct for elevation, so I put in that I basically had to hold .3 to .5 low to be on targets past 900.
I also had one of my friends watch my trace through his spotting scope so I could have some information on where my shots were going. If I don't have friends, I try to ask the spotter things like "Was I hitting high or low?" "Where were my misses?" Usually most will give you their best assessment and that gives you information to go off of.
At the very bottom is final notes. It says that the wind changed to half it's value, and that I need to make sure to true data past 1000. It's basically a final recap of my thoughts immediately coming off the line.
So what other advantages does the databook give me? It allows me to mentally prepare for the next match by re-shooting this one. Chances are if you shoot a decent bit, you are going to shoot at the same venue more than once, and the stages are going to be similar. I can look at my notes on Alabama Precision or Core and have an idea of what would make my next match better. I know that in Alabama, I would prefer to shoot the longest range stages first because the wind can get really bad in the afternoon. I know that at Core I will probably bust time on the shoot house stage, and that focusing on getting good shots is the best course of action for me. I also know that at Core, I have done pretty well on the rocks stage by just putting the Gamechanger bag on everything, and held slightly low the last time I shot, and it worked pretty well.
3 To A Mound
One of the idiosyncrasies of International Shooting is that they do not neccesarily have relays in Known Distance Matches, but instead will squad 3 shooters on the same firing point and have them shoot one after the other. This is different than in the United States because we usually shoot 1 shooter per target and then switch out the shooter. For example in the United States, you might get 20 minutes to fire 20 shots; whereas in the UK you and 2 other people are going to get 60 minutes to fire 20 shots a piece. This means that overseas you cannot wait for a condition that you like and then shoot fast through it; You are going to be on the firing line considerably longer and forced to shoot through conditions that you do not want to.
Great Britain typically does very well in International Long Range matches because of the way they are forced to train. Because their shooters have more time on the line, they have more time to write down information. After they fire a shot, they will plot their call, as well as the windage they used, and the windage they should have used. And then they'll also plot a graph of the wind they should have used because this can help them recognize trends and prevailing conditions. They might recognize that if they just leave 2.5 MOA on the rifle, they wouldn't drop any points at all because the high and low values will bracket the 10 ring.
Building Up and Letting Off
Another technique that can be applied on Known Distance Ranges and to a lesser extent, on Field Ranges, is recognizing the wind trends. Picture this; we are at a 1000 yard match, the wind is coming from roughly our 10-o clock, but we notice that it really fluctuates to almost a 1130...not quite 12. It's about lunch time at Camp Perry, so the wind is pretty close to what it's going to be for the rest of the day. There will be some pick-ups and let offs as well, but our main concern is going to be that value change.
Everyone imagines that a dead center X is the ideal shot; that's not always true. As we start out our string of fire, we notice that we have a wind pretty close to it's max value, and that if anything is going to happen in the next 20 minutes, it is probably going to slowly switch from a 10 o clock wind to an almost 12 o clock wind.
As we start our string we are going to try to build the group on the right side of the 10 ring. We aren't trying to shoot center-x's, we are hedging our bets that if anything, there is going to be a value change towards the left side, and as we start seeing this it's going to still keep our shots closer to the center. That doesn't mean we stop calling wind, but rather that we recognize that our most likely cause of heartache is going to be that value change, and we are going to try to correct for it when we can, but we know that we cannot always be perfect, so having that extra space to the left for when the wind becomes effectively less is going to save us a few 9's, and possibly the heartache 7 or 8.
The opposite could be true if you were expecting the wind value to increase from your start point. If you take your sighter shots in the low end of the condition you might want to build into the wind so that as the value or velocity increases, your extreme shots are not pushed out. However typically it's not a rapid velocity increase that gets shooters, it's either a rapid drop, or a value change.
Target Reference Method
This goes back to the data-book, and writing everything down for field matches. Targets at known distances are going to have known dimensions. The target size in centimeters or inches isn't important; knowing how wide the target is in miles per hour of wind is important though. Let's use 2 quick examples from PRS.
Example 1 is the PRS Skills Barricade Stage. If I recall correctly, it's a 10 inch target at 400 yards. If I hold dead center, I have 5 inches of left or right leeway, or for the sake of simple math, about 1 MOA because it's a round target and we are probably not going to be perfectly centered for elevation, and it's a round target. And it keeps our math simple. At 400 yards, my generic 6.5 Creedmoor load is going to take 6 miles per hour of wind to move it off the plate. In theory; If I know nothing about the wind, I can shoot in the center of the target with good fundamentals and hit every time if the wind is coming from any direction at 6 miles per hour or less! Guess what happens if we have even a slight bit of awareness and we can tell a direction? Our error space just effectively doubled to 8 inches (We are still saying it's 8 because it's a round target so anything above or below dead center will be less than 10 inches across.) If we have a pretty steady wind from the left (because we think it's greater than 6.) We can still hold the left edge of the plate, and it can go from anywhere between 6 to 18 miles per hour and we should still be on the plate with good fundamentals.
Of course PRS forces the position to be less than perfectly stable, so we are not always going to hit, and it makes sense to call the wind as precisely as possible, so for instance, in a 6 mile per hour wind that I am going to hold the edge of the plate, and mentally tell myself "Do not break anything to the right of the edge", If I am going to break a less than stellar shot, I am going to try to break it opposite of the direction that I think the wind is going to go. This is a practical application of Building Up and Letting Off.
Example 2 is a Long Range Stage at Alabama Precision. They have a Walk-it-Out type stage where you shoot at full IPSCs at 800-1200 yards from prone. We have a more stable position on more challenging targets. If I recall, that is a 20 inch wide target. For the sake of simplicity, I'll use 800 and 1200 yards as my reference targets. In a real match I would probably consider 800 1000 and 1200.
At 800 yards, it is going to take a 4 mile per hour wind to move us off of the target if we aim center. If we aim on an edge of the target, we only have to be accurate to within 8 miles per hour to score a hit as long as we know a direction. (We might actually reduce our wind reading brackets to 3mph and 6mph to compensate for errors from group size) We have a lot of error space to work with; especially at 800 yards. Anything between 1mph to 8mph and we could theoretically hold the edge of the target. Even if we have wind that is greater than the target width, knowing how wide the target is in mile-per-hour of wind can be a helpful reference point.
What we are going to do is try to land our easy hits on our 800 yard target and get wind confirmation off of that. Ideally, we want to have a blank target, and be able to watch our trace through our scope. We are going to shoot our 800 yard target and try to quickly gather data to use on our far targets. Where did the bullet impact? Did you see the trace? Did the target twist to the right? Did it twist to the left? All can be used as indicators.
Hopefully we land a shot on the target on our first go and it tells us if we over or under doped the wind, and we can make corrections from there. If we have a wind from the left, and it hits the far right edge of center, we know that we under-doped the wind by roughly 4mph. If we hit left edge we under-doped by 4mph. If we hit right of center and even with the neck we are off by about 2mph. Off one edge or the other, and we have made significant errors and should make an aggressive correction.
Now let's look at our 1200 yard target. It's still 20 inches wide, but now it's going to only take a 3 mile per hour wind error to push us from the left edge to the right edge of the target. That is why we want to pay close attention to what our refined wind-call is at those closer range targets because chances are the wind is going to be very similar. You might need to do some quick mental math and add that +/- 4 mph from your previous distances.