I can tell you what I use for my Italian made copy of a Hawken rifle. It is .54 caliber. I use lubed bullets , not balls. The bullet I use weighs 425 gr. The powder charge I use is 90 grains of FF powder. I use real black powder which is what I prefer. The rifling for my gun is a compromise between a slow 1-60 inch twist as compared to a fast 1-32. The twist rate is 1-48 for my gun. Guns can be very different as far as performance even though they may be the same model, caliber etc.You may want to write the manufacturer and see what they recommend as a starting load and a maximum load. I would guess, but just a guess, that it will be somewhere between 65 and 90 grains. Practice will tell you as well as some other ideas given here. Hope I helped and didn't confuse you any.
Back in the early days, rifles were made in workshops, no two alike. To determine the most economical powder charge, a new owner would pour in what he thought was about right, then fire the rifle - - holding it level with the ground and about two feet above a field of newly fallen snow. If there was a comsiderable amount of unburnt powder on the snow, he cut down the load until there was just a very tiny amount. If no unburnt powder is found, the rifle is not being used to maximum.
I don't know wouldn't take much advise here seriously.