Model originally uploaded to Thingiverse at https://www.thingiverse.com/thing:4299815.
Got tired of grabbing the calipers and balancing them while demarking 120mm on my filament so--here's a 120mm block that you can slot your fil into for a quick and easy marking. If you also forget how to calculate your M92 the equation is on the side of it.
Or for the nitty-gritty: https://mattshub.com/blog/2017/04/19/extruder-calibration
Each notch on the side is also 10mm so you can get a pretty good guess of how far off you are as you are testing.
Highly recommend you calculate your esteps with each filament. You don't need to re-set your E value but you'll know how much to offset your flow, which is pretty darn helpful in getting qualitative prints from finicky fils. ---yea I'm looking at you PETG..