Multiply your barrel length times 2.2580 to obtain the MAXIMUM EFFICIENT Powder load your barrel will burn.
That is NOT the maximum powder charge that can be safely fired in the barrel and gun, but its a reference place to use when trying to develop an ACCURATE load for your gun.
BP hunting is all about Accurate placement of your ball in the game- Not velocity. The open sights on your gun limit the yardage at which you can hold your sights on game and be assured of hitting within a couple of inches of where you aim. Most deer are killed inside of 50 yards, so using a ML rifle such as yours does not handicap you much at all.
In my 39 inch barreled, .50 cal. Tenn. Poor Boy rifle, I shoot 75 grains of FFg powder behind a .490 PRB when hunting deer. I can load more powder in that long barrel, but the load I settled on is the Most Accurate Load I have developed for this gun. For target shooting to 50 yds, I shoot 60-65 grains of FFg powder behind a PRB. I am using .015" pillow ticking patches, lubed with Young Country 101 Lube( Now Wonderlube, Bore butter, etc.)