Z Height Machinist Gauge Mount

When you have dual independent extruders, you can measure the differences between them to ensure the heights are the same.

I'm using a inexpensive Fowler gauge found at this link.
https://www.amazon.com/gp/product/B00B5HIXXI/ref=oh_aui_detailpage_o01_s00?ie=UTF8&psc=1

Certainly needed when using both extruders.