Set the tab of the gauge holder into the slide so that the hole faces down its length. Push it back against the cross brace and glue or acetone it into place.

Give the nozzle at least several mm head start toward the plunger. When the plunger has barely moved, zero the gauge and move the nozzle 20mm toward it and see how far it actually moves. Move it 20mm back and the gauge will show how much backlash is in that axis.

Multiply 20x the steps/mm for that axis in your firmware. Divide that number by the measurement to find the correct steps/mm.

This jig is pretty simple to make, and it shouldn't be hard to design one for any other size of platform and dial gauge.