This is a small ruler I made to help with calibrating my extruder steps/mm. It has a slot which you can snap the filament into to get a nice straight and accurate measurement. It is 120mm long to allow a little extra in case you think you might be over-extruding. The filament is a fairly tight fit but that means the ruler won't move or slip and it keeps the filament straight.

The 3mm ruler is slightly thicker and is designed to take 2.85mm filament. If you need one made for full 3mm filament let me know.