Update.......uploaded a quickie print file for those just wanting to check it out and make a quick measurement. You can use a small length of 1.75mm .068-inch printer filament for the measuring rod. Whittle a point on the end.

This will measure radius from 1/8th inch up to over 3 inches. You can use calipers to measure either from the corner of the triangle or measure what is left on top of the 3/64 inch rod/drill bit. 1.5 inch rod minus .3421 (should be) corner width minus .5 left on top equals .6579. Then divide that by .414, and you've got your radius.

It will give an exact or very, very close measure of radius.

Just use the whole numbers and first 3, and sometimes the first 4 fraction digits will give the closest measurement.

Use a 10-24 threaded screw for the thumb set screw. It will cut threads in the plastic for itself.

Made strong to prevent warping of the triangle. Must be square. Slow printing for the top thumb screw boss, or print something taller while printing this.

Use calipers for the best measurement.

Enjoy, thanks, Rick.