With the scope and the barrel light both on, rotate the scope in the rings until the vertical crosshair bisects the center of the bore light.

When it is correct, remove all the slack from the ring screws.

Keep the flashlights on while you take up the slack, to ensure the reticle doesn't move while tightening.

When this is complete, you have a scope that is level in relation to the rifle bore. The vertical of the corsshair bisects the bore axis. This ensures you do not induce any windage error on elevation change.

If, after you correctly use this method, you shoulder the rifle and the reticle or the scope appear canted, then you have a reticle that is rotated a few degrees off in the scope itself.

A tall target test will prove if that is the case, and the scope has to go in for repair or exchange.