Standing offhand stance depends upon weather or not you can use a sling for support as they differ in how the rifle is held by the front arm. I'm over 6' and on the thinner side as well so this may apply to you.
1) NO SLING ALLOWED: NRA silhouette and CMP rules for standing rimfire and highpower do not allow the use of a sling, and in at least silhouette you cannot use the magazine for support either as illustrated in Hlee's excellent previous post #4. I shoot rimfire and highpower silhouette and my pose is pretty much a copy of that picture except I have my elbow higher on the hip and use spread fingertips across the stock in front of the magwell and a thumb on trigger guard to hold the front of the rifle. Center of gravity for my rifles is pretty close to the front of the magwell. Fingers do get tired if held for 15+ seconds, and for me that happens about every 5th shot or so. I just go back to rest and then start over to rebuild my stance otherwise wobble becomes too much from finger fatigue. I dont think NRL cares if you use the mag for support or not. The guys that cannot use the hip as support (differing body mechanics) pull their front arm towards their front side basically resting it on their chest pectoral muscles. This uses more muscle as opposed to joint locking but we have guys shooting AAA and master using this technique so it can work too. You have to find out which one works for you.
Notice in that pic that elbow is up high at 90 degrees as that sets up a pocket in your shoulder socket for the rifle butt to fit into, varies from person to person but it is there. Also notice how high up the stock is, just the bottom of the butt is in the shoulder pocket. This definitely differs from bench or prone on how much surface area the stock has touching your body. You'd think that you'd get a pretty good bruise from that little area being the only contact point for recoil, but truth is I get very little bruising in standing position, I get a lot more from the bench position or shooting shotguns. 2 big tips that helped me a lot in silhouette was getting a stock or stock pad with a proper cheek weld, that sets up the 2nd tip of only moving your head forward and down when sighting down the scope as opposed to cocking it over the side of the stock---you cock your head sideways over the stock and it screws with your center of balance and stability suffers. Took me almost 3 years to put that into effective practice even though lots of experienced ppl told me about it.
Footwork: you change windage of your stance by planting your front foot and pivoting on it by moving rear foot left and right to change natural point of aim. Elevation changes by bring feet closer together to lower NPOA or spreading further apart to raise NPOA. I start by having my feet shoulder width apart and body bladed 80-90 degrees to target and adjust to suit.
2) SLINGS ALLOWED: Hasty or Hasty hasty (quick hasty) allow you to put some the rifle weight onto the sling and set up more points of contact for rifle to body. this allows for more control of rifle wobble and shrinks it considerably vs no sling. Sling will move your hand further out on the forearm of the stock as you almost feel like you are pulling the rifle into your shoulder as oppose to no sling techniques that just support the rifle at level. Fingers just lightly grip the forearm, not putting much grip ,if any, into it as the forearm should be laying along the lifeline of your palm. Same elbow, pocket, head and foot positions as above for non-sling use.
I'm not going to go into how to properly sling up as I don't feel that I could get the ideas across in words very well. What I do advocate is getting some live training. I'd highly recommend taking a Project Appleseed
https://appleseedinfo.org/ 2 day course as they are heavy into sling use for all positions and they teach you how to sling up, breathe, NPOA, and body mechanics. They are really reasonably priced and will work at your skill level.
I like Magpul's RLS Rifleman loop slings. They work like a modified ching sling, kind of a cross between a 1903 leather and G.I. cotton sling. They are easier to get into and out of then GI or 1903 slings and are wide enough to give support. Only 2 downsides I've found with them is they are not as easy to adjust for length as a GI (but once you have it set up correctly there isn't a need to do so) and if they get wet the web will sometimes fold over on itself under the slider if it isn't pulled exactly straight. They also have enough extra length to add in a extra swivel set so you can have hasty config length and sitting/prone lengths without having to adjust each time. They are inexpensive at $20 or less on sale and at that price I've gotten several dedicated and preset to one rifle. My next favorite sling is the unpadded VCAS as it also works in most positions for support---don't get the padded one if you want to use it for support as the padding wont allow you pull enough slack out of it.
Magpul RLS sling with 2 rear swivel sets. long config for using prone or seated/kneeling middle QD swivel unattached
short config for using standing with hasty technique, middle QD swivel attached
