Radius Gauge +/- 0.2mm Accuracy
The model is available for free download and use.
If you still want to invite me for a coffee for my effort, troubles and time, you can do it via:
https://www.paypal.com/paypalme/avicode
Pictures showing formula from the original design here:
https://www.thingiverse.com/thing:2746027
Unlike other designs, for this one, the zero setting should be the same as the zero setting for the actual caliper, since the caliper fits into the tool fully, and there is no extra length to be considered!
Easier math!
Also, for simpler math, the distance between the 2 prongs is intentionally set at exactly 75mm and no decimals!
Formula 1 (For radius that fits within the 2 prongs):
R = d x 2.414 (also engraved on the part)
Formula 2 (For radius that fits outside the 2 prongs):
R = (75 x 75 + 4 x (75/2-d) * (75/2-d)) / 8 / (75/2-d)
Update: Uploaded a new STL : “Radius Gauge - With Wider Pin Hole.stl”
Because one make mentioned that the pin for some calipers couldn't get out of the original hole.
This is 6mm wide, instead of the original 4mm, 4mm in thickness instead of the original 2mm.
The author marked this model as their own original creation.