A joint calipers is a simple tool to get the measure of a part, then transfer that measurement over to a ruler to get a number.

This one is sized for a 220x210mm print bed. It's all in OpenSCAD so it is easy to modify to suit yourself.

Print two of the BigCalipers.stl - I use 0.3mm layer height for that.

one Nut.stl (I used 0.2mm layer height) one Bolt.stl (I used 0.2mm layer height, 3 vertical perimeters for extra strength)