Somewhere between 35 to 42 grains of H4198 should get you what you want, you should find an accuracy node in there. RL7 would be another good candidate, though I don't know what charge to recommend.
Even with a 10 inch barrel and a powder as slow as H4895, you are going to burn all that's going to burn before the bullet leaves the barrel.
Some powders need higher pressure than what is generated by some loads in the .45-70 to have an efficient burn, the 4895s are one of them. You can't stuff enough in the case to get high pressure. An inefficient burn is the result, though accuracy may be excellent.
