[QUOTE=rjackh;537522]. Someone else recommended putting the mount on the gun once its level, then put the scope in the mount and use a plumb bob at ~20 yds to make sure there is no cant.]/QUOTE]
That method works to assure the scope is square with the rifle.
Cant error in shooting is primarily about how well the shooter can judge and hold the rifle vertical when they take each shot. In some terrain cant is easy to judge. Sometimes it can be misleading. For short range shooting can't has only a small effect on shot placement. The distance to the target, and the bullet trajectory determine what the change in point of impact will be
This diagram gives an idea of how cant affects the point of impact, but it doesn't show its magnitude as a function of distance or the bullet's trajectory. The center dot indicates the target and the point of aim. The top row of dots indicates where the centerline of the bore intersects the plane of the target. . the bottom line of dots indicates where the bullets impact in the plane of the target.. Of course no one cants a scope as much as this diagram shows, but the effects can be large at long range. it's not hard to calculate graphically with the data from a ballistics program but I don't know of a ballistics program which does the calculation and displays it. Several companies offer bubble levels which are either built into rifle scopes or attach to them. That's the most practical way to eliminate cant other than using a bench rest.
Careful scope mounting will make the effects of cant more predictable but it doesn't eliminate it or significantly reduce it.