My .62 fusil measures a true 0.620". I play with different combos, but generally prefer a 0.018 pillow ticking patch and 0.595 ball. Bigger combos are too difficult to load after the fouling builds up, though I can use a .600 ball if I swab between shots. Sometimes I'll back the patch size down to 0.015 with those balls. I usually use a 0.600 ball for hunting to have a little tighter fit in a clean barrel. A .605 ball is just too big no matter what the patch. It loads hard in a clean barrel and becomes impossible to get down with any fouling. The only time I ever broke a ramrod was trying to get a .605 ball down the barrel on the 2nd shot. I'd like to try a .590 ball, as even the .595 becomes difficult to load if I don't swab at all when shooting. Jumping between ball and patch sizes, there's no change in impact from what I can see.
As far as the charge, I like 70 grns of 2f, but use 3f as well. Any less, and the ball hits low at 50 yds. (No rear sight on my gun.)