Taping the CD fixture to the CNC 3018-Pro’s raised platform solves the repeatability problem by putting the CD at a fixed location relative to the machine’s Home coordinates. The next step puts the XY=0 coordinate origin at the exact center of the platter, so the pattern comes out exactly centered on the disc:
The fixture has a central boss:
The blue boss centers the CD’s hub hole, the red plateau supports the disc, and the white background lies 5 mm below the CD’s upper surface:
Yup, red and blue Sharpies FTW.
The bCNC probe camera image includes two faint cyan rings centered on the crosshair:
Set the diameter to 15 mm (or a bit less), center the outer ring on the hub hole = the border between blue & red, set XY=0, and it’s within maybe ±0.1 mm of the true center.