Mike, unfortunately there is little to no information in the primary literature (private diaries, journals, or letters) on how the old timers determined their optimum powder loads or how they sighted their rifles in. There is thought, though, that a ball in the palm completely covered with gun powder was a good, all-round charge. Others swear that firing the gun over snow or a white sheet until unburned powder shows up is the way to determine the optimum or maximum charge. Perhaps there are other ways, too.
What has been learned is that for a 54 cal rifle with a slow twist to the rifling, a heavier powder charge is better for accuracy, while a lighter charge of powder is more accurate for a slower twist in the rifling. To determine the twist of your rifle, if unknown, run a tight fitting patch on your cleaning jag to the bottom of the bore, mark a common reference point on the muzzle of your barrel and ramrod, then slowly pull the ramrod out while letting the rod twist with the rifling. When the patch is at the muzzle, determine the rate of turns to the ramrod's/muzzle's reference points. For me, my 54 cal flintlock rifle with a 42 inch Green Mountain barrel has a 1 turn in 72 inches rifling. That is a slow twist. I have found that my rifle groups better with a hunting charge of 90 grains of 2FF Goex black powder, which I shoot all the time whether I'm shooting paper, iron, or hunting. Rifles with a 'gain twist' to the rifling will behave differently.
I suggest that you start at 25 yards with a charge of 55 grains of 2ff gun powder. Shoot groups of 5 balls each, and then increase your powder charge by 5 gain increments until you begin to see your groups tighten up. Continue increasing your charge by 5 grains until you see the groups start to open up. Now you'll have a range of powder charges that provide you the best accuracy for your rifle. Within that range, pick the medium charge, and shoot for groups at 50 and 100 yards to see how your rifle shoots. Remember though, that at those distances it can be difficult to get as tight a group as you got at 25 yards because the iron sights make it difficult to hold your sights on the same point each shot. Also, any movements from our breathing, heart rates, and tiredness will be amplified, as well as wind and sun glare off the sights. In my case, at those distances, I typically can only see either the rear sight, the front sight, or the target with clarity due to my age and eye conditions but not all three at the same time as I could 30 years ago. Typically, I focus on the front sight and do well.
These are just my thoughts. I'm sure other members will also have some excellent insights for you.
George