As above...
Get your rifle planted level,,,
line it up with a """straight wall"""
crank optic down,,,
place cross hairs on the edge,,,
now crank it up,,,
once the cross hairs reach the top they should still be in-line with edge of the wall.
Test with live ammo,,, use level to square up the target...
Zero at bulls-eye,,, 15 clicks L,,, 15 clicks up,,, shoot.
Crank 30 clicks R,,, shoot.
30 clicks down,,, shoot.
30 clicks L,,, shoot.
15 clicks up,,, 15 clicks R,,, the cross-hairs should be back at bulls-eye...
If the box is not square & level,,, the optic needs a small turn.
Return to the wall to confirm...
Life is just that simple.