Re: Camp Pendelton 1000 yard match
It's really hard to say how you might do without knowing what you'll be running in the matches, or how the wind will be. Wilcox range is like several football fields long and very open from west to east. Seems like this time of year the wind shifts can be pretty dramatic (like 8-14 mph shifts of 180 degrees every third or fourth shot).
My suggestions for the match would be first to know your muzzle velocity for the load you intend to shoot as accurately as you can. Then you can plug the numbers into JBM Ballistics (online) and get a pretty good drop estimate for 600 yd with your setup. Just check to make sure the JBM values pretty closely match what you already know is your drop at 300 yd. With a decent MV value, there's no reason you should be off the target on your first sighter at 600 yd in terms of elevation. Then you can use the shot marker to get dialed in where you want to be. Remember that there are unlimited sighters in the first match. Realistically (time-wise) this means you could maybe get off 10-12 sighters or so and still have plenty of time to get your 20 shots in for score.
As far as windage on the first shot, you can leave most of your input data constant at JBM and use increasing wind velocity to see how far it will deflect your shots at 600 yd. I or anyone else at the match will be happy help you to estimate wind speed before your first time at the line if you need it. The main thing is to get on with your first sighting shot so that you can use the shot marker to dial it in before shooting for record. If you're completely off the target, obviously that makes it much more difficult to get dialed in.
So back to scoring...405/450 is exactly 90%. At 600 yd, the top shooters will likely do quite a bit better than that unless the wind is really nasty. But a 90% may well not have put you in last place either. However, you really shouldn't be too concerned about scores. Come out, have a good time, and you'll definitely learn some things that will help you do better the next time. IMO, the main thing is to enjoy the experience, especially your first time out. Everyone is very friendly and will try to help new shooters, so don't be too concerned. Just come on out and get started, I think you'll enjoy it.
The wind is the major challenge to shooting a good score at Camp Pendleton, so a spotting scope that can be placed next to you where you can easily look through it between shots will help you see wind changes via the mirage. It's an extremely useful technique in addition to just looking at the wind flags, which don't always read true, or can be slow to reveal sudden changes.
As far as other things, it will likely be pretty warm next Sunday, so bring plenty of fluids, sunscreen, ear/eye protection, a cap of some sort with a brim (protects you from getting hit in the face by a shot marker spindle when pulling targets...it definitely can happen!). Also, bring any food items you may want to snack on during the match. You're probably already familiar with all the other stuff you normally use to shoot (mat, data book, ammo/box, etc.). If you have a cart of some kind, bring it because you have to haul everything you bring quite a ways from the parking lot to the firing line.
Hope that helps and I look forward to shooting with you next week. Good luck!