Use this:
http://www.shooterscalculator.com/ballistic-trajectory-chart.php
Or go download Strelok on your phone.
If you don't have a chronograph, borrow one. If you can't borrow one, then go look up the estimated fps and bc for whatever rounds you're shooting.
Plug those numbers into whichever calculator you're using (I personally like strelok because I can more easily take it to the range with me and save data), and that will get you the information you want.
For example: I have a 13.8" barrel (pinned) on my AR, so it kicks out wolf/brownbear 62gr HP/SP ammo at 2750fps with approximately a 0.30 BC.
So I went with a 170yd zero. Why? Because this puts me 0.75in low at 25, 1.10in high at 100, and 11.06in low at 300yds. I've also used a 25/300yd zero, as well as a 50/200yd, and a dead on 100yd zero. Personally strongly disliked 50/200 and 100yd for my type of shooting, and while I liked the 25/300 it does put you quite high at most shooting distances.
Due to not only my ammo choice but my barrel length, my results are going to be notably different from yours, and plenty of other people's
Or just use a 25/300 or 50/200 zero and call it good.