CoreXY Belt Tuning Cube

This is a belt-tension tuning “cube” (only a third as tall as it is wide, to save filament) for use with the directions I found at https://reprap.org/forum/read.php?397,530210.  It's pre-rotated 45° for proper orientation and has labels on the sides that you should measure. Usage tl;dr: print this and measure ΔA and ΔB.  If ΔA>ΔB, tighten the belt driven by the B motor (or loosen the belt driven by the A motor).  Do the opposite if ΔA<ΔB.  Repeat until ΔA and ΔB are fairly close (the forum poster suggests 0.05 mm is close enough.  Once that's done, proceed with the rest of the calibration steps.

One thing you might want to do is to set X/Y/Z steps to nominal values for your hardware and disable bed skew compensation.  For my printer running Marlin, I added the following to the beginning of the gcode before uploading it to the printer:

M92 X320 Y320 Z1600
M852 I0 J0 K0

Once the belts are properly adjusted, you can recalibrate these as needed.