Get a 0 MOA scope base. Get a 20 MOA one-piece mount for the scope. Seeing as most ARs have a 0 MOA top rail, AND yo uintend to use the same scope on multiple guns, this gets you a 20 MOA rise regardless.

To get the repeatability that you're desiring, be prepared to spend a decent chunk of change to get a mount that has the repeatability when the scope travels between guns.