[Gaz64]

How are we verifying the "true fuel level"?

With the external tube method, the level can only be correct if the carbs are vertical.

The carb main well is on the same plane as the throttle shaft, so tilting the carb up to 10 degrees should not make any difference to the fuel level in the main well.
The main well should always be at an average fuel level, since it is in the middle of the bowl. An external measuring device will be inaccurate if the carbs are canted.

Of course, like many carbs, you can experiment with float level, some of Weber's level specs have large differences.

Gary