This centerfinder for use on cylindrical objects was designed by me. It is designed for a maximum 40mm radius but can easily adapted and printed for up to 180mm radii.
 
I included an offset of 0.1mm for a 0.2mm sharpie thickness.
All parameters can be adjusted in the thingiverse customizer.
 
Also, thanks to Rudolf Huttary for writing this awesome time saving lib which I needed to include directly in the scad source because customizer isn't able to do so itself.