there is a super awesome PDF you can download from a few places on the web that is free and gives really detailed step by step instructions on this particular carb rack.
As for this measurement, it's the highest point of the float, or, if you imagine the carbs hanging from the manifolds on the bike, it's the point of the float that touches the liquid first, because as the liquid then rises, so does the float, and your measurement (on all these carbs) is to be in charge of precisely determining WHERE that happens, and that it happens identically in all 4. Capeesh?