When I come from behind (the airplane, you sick os) I hold off til I'm inside the distance indicated on his plane of "30", closer is a lot better and the MG bullets will sink in then, cannon, which you don't have to much of, maybe "40" but just a very short burst. I've started to watch this but it is tricky aiming and looking at your gauges... Make sure the ball is centered, this means your shots will go straight, not passing to the side. Its like aiming a rifle, you have to use the back sight (Ball) with the front sight (Crosshairs). This is a lot to check and the enemy is likely to veer off after all this setting up, but thats the way its suppose to work. Then there's the spray and pray method, I use that often too.
Hang in there, it'll come, sooner than you think.
Pooka