Since making this tool, I've gotten smarter, better and more awesome :D cough And apparently arrogant.
The old picture of the tool showed me measuring filament at my extruder drive - this is wrong. The correct way to actually calibrate your extruder is to unscrew your Bowden fitting from the hotend side and calibrate into free air, otherwise your motor won't be calibrated right. After that, you need to use a digital caliper to calculate your flow rate. The flow rate is the right way to offset different extrusion for different filaments, but the motor should be calibrated without any resistence from the nozzle to give all your materials the same correctly calibrated starting point.
This tool is still super useful for this, but certainly the now deleted picture was no good.
Sorry to anyone who inherited my wrong way to calibrate your extruder motor.
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 Bowden tube fitting where the filament comes out, guide the extruded filament into the slot and use the measuring marks on the stick to measure how much your motor actually moved.
For those who prefer to make marks at the extruder frame side, the little "window" at the end will let you make a mark at 120mm in seconds as long as you angle your marker against the edge of the little "window".