How to Measure Nozzle Opening Size (accurately)

It can be hard to tell how big your 3D printer's nozzles are--the holes are very very small and can't be measured easily (0.2-0.5mm). After cleaning nozzles, different sizes can get mixed together. Even if they are labeled, the labels can wear off or become obscured. Sometimes nozzle openings can become larger or wear away from using metal or abrasive filaments. Or maybe you make your own nozzles and would like to test your results.

This is a test I devised for checking unknown nozzle siz