I added a version with ruler marks as well. It's important to print this in a good resolution - 0.1mm or lower is a good idea. You'll also want to print it with 100% infill(almost no difference in used material but less risk of failure) and it's useful to run a marker across the surface to make the indents stand out more.
Thanks to Reddit user /u/AhCup for the idea!
I had a faulty extruder arm and I needed to do a million million million calibrations and got so sick of the measuring tape sliding off and being less than precise. So I made this measuring stick with a slot that fits a 1.75mm filament. The actual slot is 1.80 to allow for even low quality filament with large variations, but it still fits snugly enough on 1.75 that it's extremely useful. Just press the end against the extruder arm where the filament enters, guide the filament into the slot and make a mark through the small "window" in the stick. This way you can make a 120mm mark precisely in seconds.
The extrusion calibration I'm using is from the super talented King Nerd Supreme over at Matt's Hub:
I suggest running that calibration before printing anything ever :D