Zero distance for 75/100 Competition shooting?

Best to zero your gun for whatever distance you are shooting for benchrest. My guns are normally zero'd at 50 yards, but when I shoot benchrest I just raise the zero to 75 or 100 yards when I'm at the bench. I do the same for practice.

Typically you zero at what ever distances you shoot at and record that information ( the number on your scope ). If you have a good scope those points of impact should be repeatable.

The only other approach would be to hold over or under and I’m sure that would be extremely difficult to do accurately enough for bench rest. 

I used to shoot a lot of rim fire silhouette and I used a 6.5 to 20 leupold scope with the target knobs. Basically sight in on each bank of targets and then adjusting the parallax and POI as needed as we moved thru the course of fire

Bench rest would be the same adjust and record and then verify using the sighter targets