For example: measure from the lowest point that you can access filament going in to the machine. I then tend to mark 80, 90, 100, 110 and 120mm (in case of over or under-extrusion).
Then I will use my software to extrude 100mm of filament. Once finished, measure how much was extruded (using the markers on the filament).
Input these into the calculator below and you will get the new value to set your esteps to in the printer settings.