Again thanks to Jeffe for the tip on downloading g-code. Magic to me but it works.
I’ve added another test pattern that can indicate whether the surface is flat and level. It moves the pen in a “kissing” motion that lowers and raises diagonally like this:
The length of the line will amplify any Z variation in the surface location.
On mine, my surface is not particularly flat or level. I haven’t faced the surface with a cutter, so it is about as level as I would expect. You can see the “X” marks toward the bottom are larger than the “X” marks near the top.
This test pattern could potentially also indicate the presence or absence of backlash in Z, which would appear as asymmetry in the “X” marks. But asymmetry doesn’t automatically imply backlash because the pen might have funny asymmetrical behavior when the line starts vs. when it stops, so that has to be considered.