I’ve never adjusted that carb, so I don’t know the specifics of how the float is adjusted; but I have rebuilt and adjusted my Ford truck’s Autolite 2100 carb a few times. I can chime in w/some general guidance anyway. You probably already understand this, but the basic idea is when the float reaches a certain level it plugs off the gas inlet. So a person might think you really have to seat that valve forcefully for it to stop the gas flow at the inlet, so they set the float by pulling hard on the float, which presses the valve tip hard into the seat. But actually it takes very little force to plug off the inlet, so what the instructions are saying apparently is “don’t do that” and instead to just let hold the works at 90 degrees to what is in normal operation, which will let the float rest in its “valve closed” orientation against the valve apparatus without you applying any force. B/c that’s where the float will be at the point the valve closes. i.e. If you physically pull up on the float to close the valve, that will be too high. Now which two surfaces to measure between, no idea. On my truck it is more like Tester describes, except on the truck the float is down in a chamber so I measure from the top surface of the chamber to the top of the float. That’s called a “dry” measurement btw. I always do a wet measurement too, to confirm the level is correct. For the most part the dry method is sufficient, don’t usually have to reset the float at all when verifying w/ the wet measurement.
edit: Don’t those Webers have a sight glass so you can verify the wet level is correct when it is on the car?