This is a remix of https://www.thingiverse.com/thing:1213459
My appologies to the original designer for not including the link to his model originally, as I was unable to locate the original when I posted.
For use as a Center Finder; simply flip it over so that the scale side is facing down and then use it like a ruler/straight edge, placing it on top of the object and then drawing two or more intersecting lines. The intersecting lines will cross at the object's center point.
When using this to measure an object, please note that the markings/scale indicates the Radius (even though you are measuring across the full diameter of the object).
If you want to determine the Diameter, simply double the reading/value shown as it's Radius.
My reason for doing this was for legibility (and precision), as the scale markings for diameter would take up much more space.
Given the method used in measuring, this easily achieves a precision of better than +/- 0.5mm and it can still be used to measure objects up to 128mm in diameter (i.e. 64mm radius)
To improve the legibility of the markings & numbers, I'd recommend doing a filament colour change,
Assuming that you're slicing with a layer height of 0.2mm, your filament colour for the text should start somewhere between 3.0mm to 3.6mm and then change back to your final colour at 4.2mm
The author remixed this model.