Sometimes for holes off center I move over the screw holes and countersinks. I put the base in modified V blocks and move the bottom radius over. Sometimes I have an end mill the right radius, but sometimes I use the boring head. Then I epoxy the mounts to the receiver while looking at the bore sighter. When the epoxy is hard, I tighten the screws. I try to get a scope with the reticule in the center of the scope tube to be bore sighted within 2 moa. I try to get the mounts co-linear with each other and the bore on the sides and co planar on the top so there is no bind on the scope tube. That is a lot easier with one piece mounts. Two piece takes fixturing.