I downloaded the same file of old blueprints as 45 Auto did (zip file at m1911.org). As I read the drawing, with the slide rails horizontal, the vertical space between the frame screws is 2.878" and the horizontal offset is 1.081". The square root of the sum of the squares of those two numbers gives 3.0743", same as the modernized drawing file showed, so I think that's the right number. It's closest to 3 5/64", with is within 1.1 thousandths. The cumulative tolerance would be about ±4 thousandths. The error of 0.014 is almost a 64th, and is enough to make you widen a hole, so I would try to use the 3.074" setting on your calipers to scribe it.
