I just added a red dot sight to my 9mm AR. Is 25 yards a good range to zero it in at? Closer? Further away? I mainly shoot 50 yards and closer with it, usually 25 yards or less.
25Y gives you a good battlesight zero out to a little past 100Y using common 115g ammo.
Based on LTC Santose's calculations at the MD AR site, a 50Y zero keeps the bullet at or below the line of sight all the time and 3.6" low at 100Y.
With a 25Y zero you're no more than about an inch above or below out to 100Y. At 125Y, you're about 5 inches low.
Hope that helps,
edited to add link and update w/ more accurate numbers.