The old "rule of thumb" that I was taught was,
"Start out with a load equal in grains to the caliber; work up from there, until a companion standing off to the side of you at about 10 yards when you shoot can hear the "crack" of the rifle." In your case that means start at 50 grains, BUT ending when the rifle starts to "crack" when shot means just over supersonic speed which is around 1100 fps. Most of us I think would agree that is a bit slow for taking a deer with a .50 (of course range is part of the equation too), but a little faster would likely be better.
The "wait for a crack" portion I think is a leftover from when powder might have been scarce, or even perhaps from The Great Depression of the 1930's and the cost of every shot counted very much. Besides with an indoor range you can't really do part-II of the old rule.
In my case, I defaulted into my load. In my state the legal minimum load for a rifle when hunting deer with a muzzle loader is 60 grains, regardless of caliber. I thought that if the game warden stopped me and decided to check my home made powder measure (I was going to make one out of a turkey wing bone), I wanted to be sure if he goofed a little on the pour of the powder that there would be no way he'd come up with under 60 grains. So I tried 70 grains of 3Fg. It shot well, and if a pound of powder = 7000 grains, then I'd expect roughly 100 shots per pound. It shot well in my .50 and in my .54, and at 110 yards I found it would put a .530 ball through and through a 90 lb. doe standing broadside to me..., so that's what I use.
For the calibers from ..50 -.58 some folks like the 60 grain load, some like 70, some like 80, some like it at 90...and some like something else in between 60-90...., if it hits where you aim and cleanly, humanely harvests the deer, you're good.
LD