Re: Shimming the rear mount.
I believe that if you run "Search" on "shim" you will find at least three threads that give the equation you need to use.
If my memory is reliable, which I doubt it is because I can't remember the last time I was wrong, you will need 0.001 inch per MOA for a short action.
As mentioned, shmming a 2 peice base 20 MOA will require the rings to be lapped or else you will severely mark the scope tube.
The Smokin Fur Rifle Club