I bought my first flintlock in the mid 70s...A .45 caliber...I used this gun for about 15 years and killed around 25 deer with...My powder charge was 70grs of Goex FFF...I tested this load by putting up a 2x4 at 75 yards and the ball passed through...A .440 ball weighs 128grs...At 60-70 yards it would pass through shoulder blades, the spine and end up under the hide on the off side...
In 1990 I made a .54 caliber flintlock, a .530 ball weighs 224grs...My initial loads were anywhere from 100-120grs...About 5 years ago I went to 80grs of Goex FFF...This load kills as good as the heavier loads...The advantage of a .54 ball over a .45 is that on lung shots I get more pass throughs (maybe 75% of the time)...Frankly, I can't see any difference as to how far the deer run with a lung shot...I just have a better blood trail...
I bought a Knight Disc about 6 years ago, initially I used 2-50gr pellets and a 300gr Barnes MZ...I always got a pass through...About 3 years ago, I switched to 90grs FFF and a 250 Shockwave...In my testing the Shockwave shot flatter than the 300 Barnes, with less powder and less recoil...I have killed deer out to 143 yards with this loading and almost always get a pass through...BUT...In my estimation, they actually run a little further on a lung shot than with the .54...I use this when hunting over fields in the evenings and since I'm 52, the Leupold sure does help...
I did shoot 150grs with the 300 Barnes...ONCE!!!
After I picked up my hat, I decided not to do that again...My hunting buddy is still laughing about watching me do that...
80-90grs is plenty...If you are looking at a long range loading, I can see going up to 110-120, my rifle just doesn't group well (the inline) with that much powder...You will see more advantage to proper bullet selection for longer ranges vs just increasing the powder charge...You will also reach a point with sabots and patches that they are so damaged that accuracy goes down the tubes...