Dirtfarmer, since nobody wants to reveal their secrets, I'll take a shot so they can mock me and maybe reveal all.
Mark a datum line top dead center on the receiver parallel to the bore. Center the base on that line and mark. Measure carefully to locate the holes. Drill small to give yourself some room for correction.
Maybe something easier can be done with a drilling jig but I don't have one. If you have bases with holes you could use that as a template and mark holes using your offset.
