I'd worked out the optimal number of 50 cal and 75mm to fire per pass, but can't recall it and can't find the scrap of paper I'd done it on. It's the best way to hit just with the right amount of both in nice even numbers, so you don't spend much attention on it. A good tactic, when you can spare the ammo, is to set the B25 into a climb (auto-leveled) and fire both rear and top 50cal at the target on your way out. You can also put the B25 into orbit near an object and stand on the rudder from the side guns to get some use out of those too, but IIRC they don't have much ammo.
And like Lusche points out you're just short (600lbs' worth or ~5-600 .50cal rounds) of enough ammo to take out a vbase's hangars without bombs. But you can use the gunner positions to fill the difference.
