If you put the rifle in a secure mount, aimed at a point at the bottom of a truly vertical line, and run the vertical turret to the settings required for progressively longer distances, the intersection of the cross hairs may come off of the line. If the angle of divergence is noted, the scope can be repositioned so that when the same test is made, the point of intersection stays on the line. Once this has been done. some sort of on scope level can be mounted with the bubble leveled and secured. in the case where the vertical cross hair is out of alignment with the tracking of the intersection, once the correction has been made, there is still the problem of having to rely on a level to see when the rifle is properly aligned. In any case, doing the entire alignment with the CL of the scope directly over the CL of the action helps insure that the initial sight in distance is not at the intersection of a horizontal angle of the LOS and trajectory, which will diverge beyond that point. As I mentioned earlier, this has the further requirement that any internal barrel curvature be clocked into the vertical plane. On the other hand, I have mounted many scopes using simpler procedures with good results....large targets, moderate distances, one shot kills. On the other hand, if for an egg shoot or really tight, really long precision, I would go the limit.
