Tim is correct, just shoot it at different distances, you will soon learn what it takes for hold over/under. Like you said, zero it at 100 yards and then shoot 3 shot groups at 25, 50, 75, 125, and 150 yards. I would only be concerned about 150 yards+ if you really think you will get a shot at that distance. If you are zeroed in at 100 yards, it will be considerably low at 200 some where around 15" or so. A 150 yard zero will put you about 8" low at 200 yards and 3.5" high at 100. I am making an estimate based on a Hornady 180 grain XTP @ 2000 fps - that is all an estimate. You need to shoot them to be sure. It will not only tell you how far to hold under or over, but how accurate it is at every range. Good Luck and Good Shooting Pete.