I run 26 grains of FFFG in my pietta model 1858 remington. Using #11 remington caps, and a .451 Hornady ball, there is very little felt recoil, and I have run about 100 rds through this weapon without a single failure of any type. I am having very good groups with this set-up, and really see no reason to go any hotter. I think Pietta recommends between 25-30 grains, but check your manual, and I would probably follow those recomendations.
-Bruce