Skychief,
When I got my Mark Silver kit from Jim Chambers, maybe 20 years ago, (42" barrel ) he recommended 75 grains of 2F, and I found that worked and I stuck with it. I had been using 120 grs in a .54, but the heavier ball did the work in the .58.
Most shots went right through and I collected very few over the years. Shots to a shade over 100 yards, and close as 20 .
With yours being a bit slower twist, you may need to shove up the powder a bit. But between 80 grs and the above mentioned 120 grs, you should find a happy load.