Measure barrel max an minimum. Select a ball .010 smaller of minimum. Load 60 gns fff. Use .010 patch. If it starts very easy try .015. Don't want it so tight you have to hammer it in but not thumb press in either. Work up on the load till accuracy dwindles or just doesn't improve considering amount of powder. Black powder and substitutes deliver peek pressure at a certain load and adding more does not deliver enough more to be worth wasting the powder.
I have a 45 Kentucy, 50 Hawken and 58 Zouave. All shoot good at 65-70 gns.
When it comes to distance they have a trajectory like a rainbow. More powder will not make it shoot much flatter.
A .478 mini will probably work at same load. If the bore measures tight (under a hair) a .475