The key to accurate dive bombing is consistency. If you want to use the same sight picture you have to use the same dive angle, speed, and release altitude. This is best done by setting up your approach the same way. Start at a consistent altitude above your target. If you approach perpendicular to the target you can roll and point your wing at the target and use your bank angle gauge to get an idea of the dive angle, just allow for your head height above the wing. As Waystin mentioned 60-70 degrees is generally good, just be sure you don't get too fast in the P-38, use a shallower angle if you need to. Rudder trim is important but shouldn't be an issue in a P-38. If you use an air spawn field in the training arena, like A23, you can easily practice consistency in dive angle, speed, and release altitude. If you load the TA arena for offline training as Morfiend suggested you can damage the target when you practice.