I use a small level made by B-Square that clamps to the scope base, look through the scope find the target, glance to the right (out of the way otherwise because I shoot lefty), its there, center the bubble between the black lines, line up on the target and shoot.
getting the crosshairs dead plumb is another issue but once its done its done
RR
Oh and they are around 12 bucks each, got one on all my long range rigs