I like to use the plumb line method. I level my rifle on the bench using a bubble level across the rings without the top half of the rings on. Down range (300 yards away) I have a yellow rope about 12 feet long with an old window weight on one end hanging from a tree limb. I set the scope in the rings and rotate to line up the reticle with the rope. I also use one of the levels that goes into the bolt raceway to insure the gun stays level as I tighten the rings. This is the most accurate method that I have found.