You can get the center-to-center distance by starting with the method Mica shows above (outer edge-to-outer edge) and then subtracting the diameter of ONE of the holes. Allowing for a few ten-thousandths lost to measuring on the curved surfaces, you'll still be well within standard manufacturing tolerances of the actual center-to-center dimension.
i.e., if you take Mica's technique and get, say, 1.643, and then measure the hole and get 0.143, the resulting center-to-center would be 1.500.