I've never done this on a tractor carburetor before, but a trick I've used on my motorcycles to make sure the float is operating correctly and to set the actual fuel level, rather than the float height, is to connect a piece of clear tubing to the drain of the carburetor. Even just jamming and kind of threading it in works fine.
Run it above the level of the float bowl, and turn the fuel on. If you know the correct fuel level (in this case you could measure it from the diagram given) you can compare where the fuel sits in the clear tube to the bottom of the carburetor, and have a precise and accurate measurement of the fuel level, at any given time, even, if you've got it set up safely, when the machine is in operation.
On carburetors that have sat for a long time or are suspicious, sometimes the float level is set to specification, but it is unknown whether the float works properly, or the valve is jamming, or leaking, or not flowing sufficiently under load. This method gives you a way to check what you're really interested in, the fuel level. Good luck on your restoration!
