120mm filament marker - extruder calibration tool - 1.75mm filament

UPDATE 2:
Since making this tool, I've gotten smarter, better and more awesome :D *cough* And apparently arrogant.
Anyway....

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