Suppose somebody had a .44 Magnum and they were to use a 240 grain bullet at about 1200 fps.
On the other hand, a starting load for the .45-70 entails a 250 grain bullet at about 1800 fps.
A 250 grain .45-70 slug launched at 1800 fps will still be travelling at about 1200 fps at 100 yards.
So shooting a deer at 75 yards with a 250 grain. 45-70 is ballistically the same thing as shooting a deer at a contact range with a .44 Magnum.
So stop overthinking things. Load up a starting .45-70 load and you will have ample power for stopping a deer at 75 yards