Should be 1/8" per foot so there would be 9 1/8" difference in elevation over 73 feet.

A good rule of thumb for wastepipe carrying solids,1/4 per foot is considered optimum.

Yea but that is for 3" or smaller drains. A 4" only needs 1/8"

1/4" per foot would be optimum if you have the height to fall 18.25"