Suggest you start out with about 50 grains of FFg and shoot 5 shot groups. Increase load in 5 grain increments. Groups should tighten up to a certain point, then begin to spread out again.. The tight point is the sweet spot.
You need to experiment with patching too. Find the fired patches. If the edges are frayed,or the patch is shredded, the patch is too thin.
Patch should fit snugly when loaded, but not real tight. When fired, it should remain essentially intact and have a discolored center from burning powder and may have traces of the rifling engraved in the patch.
If you want the best accuracy attainable w/a patched round ball, and you want the simplest, most expedient, and economical patching system there is, for a muzzleloader, here's what ya do:
1. Use only pure linen w/the simplest 90' weave, .010'' to .015'' thick, depending on the OD of the ball(s) used. Use a micrometer to determine the linen thickness. Buy it by the yard, cut it into 1.5'' to 1.75'' wide, long strips, lube it w/your favorite lube, and roll it up tite.
2. Unroll a short section of the linen strip, lay it across the muzzle, the same direction every load, and start the ball into the bore, over the linen. When the ball is below the surface of the muzzle, gather up the linen over/around the ball, like a pouch, and cut the linen off, w/a very sharp patch knife, flush w/the surface of the muzzle. This will leave a round hole in the linen strip coming off the roll, and will center the ball in the patch every time. Finish loading the patched ball.
The old timers said you have to learn your gun if you're gonna make meat every time.
Thus the old saying, beware of the man with only one gun, he probably knows how to use it.
Have fun.