Mauser,
I lived in Pa. and hunted with flintlocks for many years--still hunt there in flintlock season every year.
When I began, the most common load for 50 cal round ball seemed to be 90 gr. ffg, so that's where I started. In my old TC Hawken, I got reliable ignition--after I replaced the crappy TC frizzen with a Lyman--got pretty fair accuracy and killed deer quickly.
I have two flintlocks, and both have very fast ignition. The key, I have discovered is to use a good frizzen, a good sharp flint, and not overloading the pan with powder.
Then I met some guys who shot flintlocks year round at matches, and they suggested that I try fffg. Since the grain size is smaller, and it burns faster, they recommended that I begin with about 75 grains. I have tried loads of from 75 to 90 grains, and for my rifles, have settled on 75 grains of fffg.
The velocity and accuracy are about the same as 90 gr ffg and the deer don't know the difference, but I seem to get less fouling and have the added benefit of getting more loads per pound of powder.
Oh, by the way, I still use patched round balls for Pa. flintlock season.