Convergence with the nose mounted guns are not nearly as important as with wing mounted guns. The only difference it makes is at what range the arc of the trajectory should cross through the horizontal center of the gunsight. And with that said, you could probably get accustomed to any range you choose.
My settings are 400 yards with .50s and 600 with the 20mm. I set the 20mm farther to make up for the slower velocity, and the fact of shooting at a moving target from a moving platform. I probably have 15,000 kills in an AH P-38 for what it's worth.
