I use IMR Trailboss for this exact purpose. Simply make up a dummy round and cut the case just beneath the bullet. use that cut off case a sa dipper for your powder and measure out 10 charges, unpacked and struck level with the top (i.e. the powder would be sitting just below the seated bullet in the case) then take and average weight of the charge and reduce it by 10%. use that as your load. Trailboss runs its best consistency at 90% load density and Ive done this very procedure with many BP firearms with UNcatasrophic results.
Good luck speedy!