Well.. if you set your convergence to 350 yards or so in the P38 (which I don't even think is nescesary, unless it effects bullet drop), you can put your nose on someone flying straight and level and hit them from probably 500 yards away.
Understand, however, that the .50 round is not the most damaging round in the world, and it will take more than a few to bring down an enemy plane. This is especially true at 500 yards or more, because Aces High DOES model energy loss, so a bullet that has to fly 700 yards to hit a target isn't going to hit as hard as a bullet that only has to fly 300.
I'd recommend holding the distance to less than 300 yards until you are pretty familiar with the way the guns fire. Also, your P-38 isn't the best plane in the world for taking snapshots with, although with the single cannon it should do it better than a P-51 would. You can contact Fester, or Tac, in the MA.. both of them are excellent P-38 pilots, and I'm sure you could get one of them to show you the ropes, so to speak.