I zero my red dots on my ar15's (16.5" barrels) at 28-30 yards, that crosses zero again at around 335-350 yards. The military typically uses a 25m/300m (I convert meters to yards usually) zero. Which is essentially what I try to do with my red dot guns. It really does come down to the following though as to where exactly you zero it to do this.
Even if your shooting factory ammo you can get the bullet info from the manufacturer or off the box. If you give me the gun and the barrel length I may be able to estimate the fps.
Scope Height from Barrel bore center=
FPS through Chrono=
Ballistic coefficient of the bullet=
Elevation=
Bullet Diamete=
Bullet Weight=
