How to adjust rig tension with the folding rule method

According to my selden manual should it be possible to adjust the shrouds to the right tightness with a simple trick called "the folding rule method" where you tighten the wires untill they extend to a certain length.
Always easy in theory but not so easy in real life.
A good tip is to tighten up the backstay if the scroud screws is to tight to turn, the backstay will take some of the pressure of the shrouds.

Видео How to adjust rig tension with the folding rule method канала Tino G - Sailing