There several contact surfaces that come into play, the receiver, rail if you use one and the rings themselves. I always carefully check the rail to receiver fit first with a really straight edge and often bed the rail to fit the receiver. I take some blue painters tape, cut out a small area to expose the mounting screw holes and just snug the rail. Then take an exacto knife and carefully cut the line around the base of the rail. Remove the rail and the center portion of the tape and you have a very clean line to protect the receiver from bedding compound or JB weld. Put a liberal amount of release agent on the receiver, rough the bottom portion of the rail which is high, clean it and apply you compound to the underside of the rail. I plug the screw holes (both receiver and rail) with clay putty, snug the screws on the opposite end which should squeeze out a bead around the bottom of the rail and clean the excess with a q-tip and a little acetone just on the surface of the rail. Let it cure, remove the rail, clean the hole and torque the rail to spec. Now you have a flat surface on which to check your rings and if they're not perfect, I do the same thing with the lower half of the rings. There's a good article on this site so I won't go into that.
