I have a .69 blanket flintlock and a .75 brown bess. I was taught that to use 3f from .69 and smaller and 2f from .69 and larger and either or in the.69. I have always had good results from this rule.I use 44 grs FFFg in my .58 Enfield with good results on breakable targets at 50 and 100 yards. This pushes a 500 gr minie.
60 grs FFg was the standard powder load during the Civil war, with devastating results.