Diameter Gauge

This Thing is a Diameter Gauge, inspired by SubSite's Radius Gauge. For my purpose it was more appropriate to measure diameters, so that's what I've got; no real difference (though it simplifies the equations a bit). Also, I needed to measure somewhat larger diameters, up to 36" or so, so my gauge needed a larger span, to improve accuracy on diameters that size. This has a 200 mm span, which is the largest my printer could handle, with the part placed diagonally.

Then I noticed that with a