It's documented on the UK site as:
Fuel level 3mm below the carb mixing chamber body edge or gasket.
I don't know if that helps or not?
I think the easiest way to set height to those levels is to mount the carbs in a vice and set them level using a spirit-level. Other people have drilled through a spare carb drain screw and attached a length of plastic tubing that can be held up against the side of the carb as a gauge.
Like this ...
http://www.bbburma.net/FloatHeight.htm