Pretty simple. Set bike on it and adjust the bike so the center of the spindle is in the right place. You can instantly see where your saddle set-back is in relation to the spindle (5 cm set back required) and the extensions in relation to the spindle as well (80cm from spindle center to extension ends by official Bruce’s claim). I think the confusion and headache comes from the extension measurement. Some say end of the extensions where the shifter goes in, and others (according to Ian Stanford) measure to the pivot point of the shifters. This is about 1.5cm deviation in ruling. However, if you’ve ever installed shifters, extensions etc it’s a real rat-fuck if you have to make on the spot race-day adjustments. The problem is compounded for shorter/taller people who, if their bike fails the test, have to have physiological measurements which are even more invasive and time consuming.
My bike falls 0.5 cm shy at the saddle and at the extensions meaning that i’d have to adjust everything back 0.5 cm to conform to the UCI and I’ll do that the minute the UCI sends someone over to mow my fuckin lawn.