Helps to have a center dot max, running an old CRT I can get away with putting a very small black permanent marker dot dead center. It can be trimmed or removed with a razorblade. However I suspect that wouldn't work so well on a LCD screen.
Mouse pointer trick works, lock it with the H key to prevent accidental movement.
600 and in use dead center, 800 and farther out put target about halfway between the center dot and the innermost ring. iL2's big cannons are incredibly accurate, set convergience about 5 - 650 and you can kill planes at 1.2 or more away.
Like the Yak9t you need a "1 shot, one kill" sniper mentality against planes. Take the time to line up each shot. Make sure controls are centered. Line him up and tap the fire button once. Watch where they went, adjust and try again.
Against tanks I'll often shoot all guns, trying for 3 or 4 shots on target from 800 yards in. Perhaps a bit farther out if I'm after a wirble. Wirbles are nasty for iL2's unless you can get right above them 2k up and drop a bomb on them.
