I've told this story before, but it's worth re-telling (short version). An old Army Range Sgt, back in the early 60's, told me if I ever had to zero my M1 in a lurch to do it at 25 yds and then fire for effect out to 300 yds and beyond. He said that, for all practical purposes, doing it that way would put every round inside a dinner plate out to well over 300 yds.
I tried it and he was right. A 25 yd zero was around 1 1/2 inches high at 100 yds, about the same at 200 yds and back in zero at around 300 yds. That was a long time ago, so the figures may be a little off, but the principle is solid. It really saves a lot of ammo.
I've never tried it with 7.62x39, but I'm sure the principle will work, certainly out to 200 yds, or so.