Another way to measure the length of a thread of arbitrary length is with the help of a pendulum from a telephone.
The thread must be fastened tightly. For example on a nail. If you hold the end of the thread in your hand the measurements may be incorrect.
With free swing, the application will show the length of the thread from the fixed end of the thread to the center of mass of your phone / tablet. Usually to the middle of the screen of the phone / tablet.