I’m certainly no expert, but here’s my approach. For base defense I like the ostwind because when I finally hit a plane I want to see parts coming off. I use Torquila’s sight (the one with the rings for 1k, 2k, and 2.5K). This is available from HTC home page ->community->gunsight gallery.
You will get most kills when the fighters are trying to strafe you because they usually fly straight at you. I aim with the plane centered on the 1K ring (a little zoom helps here). Start shooting when the plane’s icon shows 1.5K. You often get a hit by the time they are at 1k. One hit will almost always take care of a fighter. An IL-2 may take several hits. If they are still alive and get inside 1K, put the pipper right on the plane. Once they get this close though you may take some hits. If they have bombs or rockets you really need to kill them before they get this close.
For deflection shots, I don’t even try unless they are 1k or less. Even then, it takes a lot of practice and a fair amount of luck. Don’t use any zoom and lead them a lot. I usually aim way in front and hope they fly into a shell. If there are a lot of enemy fighters, as is often the case in base defense, I only shoot at the ones coming for me. Otherwise you can get distracted, allowing another fighter to get you.
The same principles hold for the M16. Deflection shooting is a little easier but of course it takes more hits.
I find I aim best when I’m not moving. During lulls, I try to make my to the town for any c47s coming in. Once you get moving you can drive from the turret position, where you can see better and shoot if necessary (use rudder to steer).
Be on the lookout for bombers because if you are stationary they will drop a bomb on your head. If I see a bomber heading towards me, I jump to the driver position and start moving as fast as possible – get your speed up and then maybe turn a little. I rarely waste ammo on a bomber unless they are really low. Usually this means they are out of bombs and trying to gun you. It often takes several 37mm hits to kill a big bomber.