Bad Design

It so happened, that i were to take a print...about 5500mm by length...(thats quite long!)..from the plotter at my office. The paper roll was almost exhausted...but wasn't completely empty though.
I had no clue if the amount of paper left in the roll would suffice my requirement or not!(Bad Design)
Thats when i thought of this simple yet effective solution.
Since the dia. of the spindle in known..and also the thickness of the paper...so if there is a calliberation on the wheel of the spindle...then we might be able to atleat get an approx. idea of the amount of paper remaining in the roll!