450 - what is the length of case this rifle is chambered for.
: The original .45/100 load was used in 2.4", 2.6" as well as the 2-7/8" case so a case length designation is much more accurate than by powder charge. The .45 2-7/8" was also loaded with 120gr. and was the 'original' .45-120 of the 1880's. The modern 3-1/4" case was never chambered by Sharps in the 800's, but there was a .45-120 Winchester round which used the British case. Some Sharps rifles may have been chambered to the Winchester case on the frontier.
: Your heavy .45/70 loads are a good place to start.
: Accurate Arms might also have data for the shorter than 3-1/4" cases.