I have the "standard" 16 microsteps extruder driver, so the formula for steps/mm should result in 456. Tried this (with no hotend) and 100mm asked resulted in 112mm extruded. Adjusted to 407 steps/mm. Attempting to understand the cause led me to this: The boring in the hobbed gear is not flat, therefore it is important to have the filament as centered as possible, otherwise the radius of the hobbed gear is changed (hopefully not during printing) and the steps/mm is changed too. The reason for not centered filament seems to be the black (1.75mm) guide that tries to put it off-center. I've attached a picture that clearly shows the filament "liking" the bigger radius of the hobbed gear.