Don't know what kind of powder your using. But a good starting point is 80 gr of FFg or Pyrodex RS or 70 gr of FFFg or Pyrodex P. Work up in 10 gr increments and watch group size. You may find the sweet spot right at the begining or as you work up. (groups may get tighter then begin to open back up). Finding the sweet spot may take a few range sessions so don't get frustrated.
Be sure to swab between shots either with a spit patch or alcohol patch.