So I thought I would take one more crack at this. I shot a doe at 50 yards with a 54 round ball and although I did not make the greatest shot, she was done for depsite running 100 yards (in the right direction, so there is that). I like some margin on error in my hunting setup, so this serves as a reasonable benchmark for me given that this is a proven load. I took the ballistic data for muzzle velocity for this load out of the Lyman black powder manual and plugged it into a ballistic calculator. It spat out that I hit the doe with about 700 ft lb at 50 yards. I know energy is not the be all and end all of what kills an animal and that the diameter of the bore is what helps us a lot with smokepoles, but it serves as a handy benchmark.
When I plug in a 50 cal PRB out of my rifle with 70 grains 3F, a 58 cal PRB with 80 grains of 2F, and a 45 cal maxi ball with 70 grains of 3F, I get right around the same 700 ft lb at 50 yards for all of them based on starting velocities in the Lyman book. 45 PRB with 70 grains of 3F pencils out at less than 600 ft lb, and the lower weight of the ball and its small diameter means it is in a different class of power. 45 PRB isn't legal for deer in my state, so this is really just to see the difference.
Out to 100 yards, you either have to jack up the charge or use a heavier projectile to get to 700 ft lb, but I have no doubt 500 with a big diameter and sufficient projectile weight would do it. The 58 PRB could use just a smidge more powder to get there, but probably has more than enough killing power with 80 grains. Any 50 grain or better conical would do it easily. I'd imagine 80 to 100 grains of powder with 50 or 54 PRB would do it. With a 45 maxi I would be cranking up the charge to the most I could shoot accurately.
Most likely I am taking the 58 for deer this year, but it looks like the 50 with 70 or more grains would do the job with PRB. I am not all that crazy about taking shots at deer with open sights much beyond 75 yards and where I hunt I really don't have to, so i think I understand pretty well what I need to be looking at in the way of charges.