The pic sort of explains where to measure from and to, but you have to be sure that the float valve has just shut, but the little springy bit in the end of the float valve is not compressed.

Tip the carb up so that the float valve shuts off, then slowly lower it down again until the float fall away and the valve open. Tip back again until the float valve will just stay shut. It's a bit of a balancing act, but easy to get the hang of.