I eyeball getting the rifle verticle in a gun vise. Usually, for a commercially built rifle, the stock is machined on a stock duplicator and the but of the rifle is verticle and in line with the bore. A level laid across the turret cover can then level the scope. You can usually do pretty well just eyeballing it by looking at the verticle cross hair and the edge of a building or pole that you know to be verticle. Snug the screws down a little at a time going from side to side and verifying verticle or the mount will often cause the scope to rotate slightly.