I get the cross hairs centered in the tube.
My way of rotating the scope and change the turrets until the hairs cross are in the center.
I have since read about pressing the objective against a mirror and adjust the turrets until the crosshairs line up with the cross hair reflection.
With the rings tight on the bases and the scope, I shim [with narrow shims] the bases until the crosshairs align with the cross hairs.
I then epoxy between the bases and receiver, but leave the screws loose.
After the epoxy is hard, I take off the rings and tighten the base screws.
Now centered scopes can be swapped from rifle to rifle and be on the paper.
Now the rings are not putting a bind on the scope tube.
Alternatively I can put a scope base in modified V blocks and mill out, with a boring bar, the concave bottom of the base.
I can change radius, height, or introduce and offset.... ala Steve Acker.