I got tired of wrestling with the filament and my calipers to put a 120mm mark on the filament so I made this small two part gauge. The base has a 2mm slot to lay your filament into then put the cover over it to hold it in place. The base is 120mm long and cover is 10mm shorter. Put the end of the gauge up next to your extruder and put a mark on the other end of the gauge. I use community firmware on my CR-6 but teaching tech has a great web site for 3-D printer calibration.

https://teachingtechyt.github.io/index.html