P-38 seems like a special plane. For my 109s (another nose mounted plane) I have to set the guns to 300 (my preferred setting) since the bullet ballistics are different. On the P-38 though I don't really feel that difference. I don't know if the 50's and 20mm have a similar ballistic pattern or what it is but the guns seem to fire pretty straight and accurate.
I set mine to 450, both the 50's and 20mm. Seems to work for me pretty well.