I started at 48.0 grains and quit at 50.0 because my accuracy was nice and velocity was what I wanted.
Brian Pearce did an article in Handloader a few years back on loading the 45/70 and my velocities followed his with H 322. It's my favorite powder for heavy loads.
I used R -P brass, CCI 200 primers and applied a heavy crimp with the Lee Factory Crimp Die.
Good luck with it!