This is basically the lazy/cheap person's way to level a bed without a dial gauge. When built, you basically have a clunky gauge that works exactly the same, and slides along the x-axis of your printer while you adjust the bed.

Print.

Use an M8 tap to cut a thread in the hole through the side of the part.

Insert the callipers (you may need to temporarily remove a little clip that screws onto the bottom of them).

Insert an M8 bolt/bit of threaded rod into the hole you just tapped, and tighten against the callipers to hold them in place.

Put some elastic bands around the callipers and the nubs on the side of the printed part.

Level stuff :)