well, i been working on this for some time and i was using the total used lenght of filament, then divide that betwen the total lenght of the roll, and then multiply that by the price of the roll, and it was working fairly ok, but i didnt know the extruded filament was longer, i guess i need check that, as for the way to measure i need to buy an scale, i dont have one, anything i will tell you back people