One piece extruder E-step calibration tools for 1.75mm and 3mm filament diameters.

100mm length with graduation marks every 5mm/10mm.

Tunnel has a sinusoidal tilt to hold filement in place and straight while calibrating, and still allow for the tool to be attached without unloading the extruder. Slide feet on, then bend filament slightly to follow the slot. Can be attached one-handed with some practice. Tool should spin freely on filament.

Deep notches at each end allow for filament to be marked (I recommend ultra fine point Sharpie) at the top and the mark clearly visible at the bottom.

The design relies on the printer being capable of clean 45 degree overhangs.

My printer doesn't use 3mm filament, so the tolerance is a guess on my part, but it's proportional to the 1.75mm version.