Here is the straight goods, meaning I am not guessing at anything, or repeating someone's pet theory.
For starters, a 405 grain 45-70 bullet travelling from very low speed, like 11-1200 fps and the same bullet travelling at 1700 fps, will have a poi at 100 metres difference of about nine inches.
If you want to use the popular bullet weight of 405 grains, or thereabouts, which is a good idea, then use it, loaded to the velocity you want.
Sight it in, the way you want it sighted, about on, or slightly high, at 100.
Using your open sights, adjust the notched elevating bar. If it won't come out right, sight it in the notch that is higher than you want, then file that notch down until it shoots where you want it to shoot.
After you have it sighted in, do not change your sights. Period.
One other thing. Flat base bullets will shoot in your 45-70 at considerably higher velocities than you have been told.