These bars are similar to the ones i use on my industrial sla machines to calibrate scale factors.
thought they maybe be useful for testing the accuracy of 3d printing.
Simply build and messure to see how close your machine is.
they could also be used to calibrate the steps on the motors.

there are three bars in each axis.

5x5x50mm
5x5x100mm
5x5x200mm

ive also added the axis x or y with an arrow showing positive direction so when removed from the bed you can still identify them.