Use published reloading data only.
This a great link for loads.
http://data.hodgdon.com/cartridge_load.asp
Trail Boss has been popular in 45-70 check the link for cast bullets and the Lyman book has good cast bullet info.
Also look at the Trapdoor data for cast bullets, probably pushing them faster than 1200-1300 fps might give you some leading depending how hard or soft they are.