I zero conventional cartridges like .223 and .308 at 200 yards, by sighting them in on a 100 yard target and knowing (using a ballistics calculator) how high they should be hitting at that range, with a 200 yard zero. I will then later try some 200 yard shooting to verify the zero. For my M1A, which has meters on the rear sight, I use a 200 meter zero.
The zero, of course, is independent of the sight-in range. It is where the bullet crosses the line of sight for the 2nd time. The sight-in range can be any range, doesn't matter, as long as you know what that range is. However since (I think) most scopes are set to be parallax-free at 150 yards, that is actually the most accurate range for sighting in. If I use a different sight-in range, I take care to keep the crosshairs centered in the image, to eliminate parallax as best I can.