Make tracks of your gunnery practice then watch them on playback with arcade set to 1.
arcade mode: open conf.ini with Notepad, find the line that says arcade=0 and make the 0 a 1 then save.
In playback make sure you have control of view and time. You can zip through until you're about to
shoot then slow it down to 1/4 speed. The instant it starts to fire, pause the playback and check
your sight picture and The Ball (ask if you dunno) and then change the POV to the target plane.
Slew the view around to where you see your plane behind the target and get it in close, unpause and
see where the tracers go. Any hits will show as arrows in the target. This should give you a very
good idea of how close or not your shots are going and may surprise you very much.
Also when shooting, get a feel for how long your shots take to cross the distance. When shooting
deflection you won't have to judge angles, just see how fast he is crossing your view and aim ahead
however far he would move in the time the shots take. Aiming a little ahead and lengthening the
burst (talking 1/4 - 1/2 sec longer) is a good way to cover the "I'm not sure's". It's like
throwing a ball to a running catcher, practice and you'll get the rhythym, it's like music.
Remember that your speed relative to the target changes the timing. Deflection shooting, it goes
even more since you're not on the same path and close quicker than your speed minus his.
Shooting uphill or downhill, gravity is no longer straight down to the bullet path. Always aim
low when not shooting (mostly) across. At least IRL this is true.