Flint musket cartridges back in the day were 110 grs. of musket powder (about equal to ffg, but some was actually as coarse as fg). Part of this was used to prime the musket.
I use a .662 ball over 75 grs. of ffg. This size works well with a .018 pillow ticking patch and also can be left wrapped in the cartridge paper.
You can use fg, but you will get better ignition and velocity with ffg. If you're shooting a musket with a large touchhole, fffg will blow out into the pan when you drive the ball home. If the touchhole is small enough, fffg can give good results, just reduce the charge a little less than with ffg. I have used from 70 to 90 grs. with 75-80 giving the best results.