One method that I have used if the lower half of the rings lends itself to it, is to place a level across the top of the lower half of the ring to level the rifle to make sure it is in plumb, then mount the scope without moving the rifle. Then I used a vertical plumb line 10yds away to make sure the scope is plumb with the vertical cross hair against the plumb line. I've also used a short plumb line to line up the screw heads in vertical plumb on the recoil pad to make sure the rifle is held true to plumb. Tim