Well, 100 yards is a good shot with this rifle, but the ballistics of an RB dictate that energy is shed at a high rate. In addition, accuracy may not be all that great beyond that, either: the previous replies are correct about shorter range.
Start with about 70 grains of FFg and go up to about 90 grains, testing for group size. You can shoot FFFg also and sometimes it works better, sometimes it does not work as well; you will simply need to shoot and see.
The old-timers would start low, say 40 or 50 grains, and work their way up to a point where the rifle would "crack" when shot. They didn't know it, but that was where the ball went supersonic. Powder was at a premium in the old days and they shot no more powder than was barely needed. You can do better by starting a little higher and testing for grouping.
Shoot real black. Pyrodex is more corrosive and you must always measure Pyrodex by volume, not by weight. Black isn't so fussy and will give excellent results: it is accurate, it is easily ignited, and it smells better.