According the the factory service manual diagram(pictured in post #2) It is measured from the inside of the float bowl lid.
When this thread(multiple forums) came up I decided to adjust my float bowls with a site glass. Before I was going to look into finding bungs that would screw into the drain plugs. Then I read arne's post and decided to use his method which is basically the same way the OP set his site glass up, only his sight glass is graduated making it easier to see where the level is. I used what I had from my miracle whip jar brake bleeder set up, which was a piece of clear tubing about 12" long and I marked the float bowl with a pencil where the level needed to be at. For the first time in 10 yrs when I move the mixture screws I can actually hear a change in idle unlike before when I had to turn the knobs down at least 3 turns. I just kept the mixture knobs at 2 turns down. It seemed to work, no pinging, good mpg and still a lot of fun.
When adjusting floats using the 9/16" dimension float to lid, it turned out I was 5mm too low compared to the 23mm dimension using the site glass. With the site glass I am within 1mm of stock specs. I still have work to do, reading plugs, figuring out idle settings when checking mixture, but at least I am close to be able to even use the mixture knobs.
The light colored plug was from the back carb at 2 turns down, the black one was from the front carb at 3 turns down, that was when the idle changed, both idle front and back sounded the same. To sync the carbs I used a hose and my ear. When I checked that against a unisyn, there was no difference.