This is my version of a circle center finder tool for round stock ranging from 1/4" to 7" in diameter.

The slot is positioned to compensate for the width of the marker or pencil tip There are two versions. One with a slightly wider slot for general marker use and another with a thinner slot for mechanical pencils with a 0.5mm lead.

Three lines crossing at the same point with this tool confirm the center of the round stock.

To keep from under or over extruding the print it is recommended that you thermally calibrate your filament using these clips: https://www.thingiverse.com/thing:2502677