The factory shop manuals do not use the method you picture above. You simply remove the tops of the carburetors after filling them to their set level, then measure the depth with a vernier micrometer as pictured. Of course, in this case you are measuring to the top of the aluminum body, NOT with the gasket still installed. Also, note the instruction on the 2600 to NOT bend the tab, but to change the number of gaskets under the valve body to effect the filling level.
After making an adjustment, you must reinstall the carburetor lids, turn on the fuel pump, refill the carbs, and do it all over again until you get where you need to be. Be sure there is some fuel removed from the carbs before reinstalling the lids, as they may not drain when you reinstall the top and give you a false "high" later on.
In practice, a little variation from the 28-30 is not going to shockingly change the behavior. I just had a leaky float in a Weber that ruined two cylinders, but a millimeter one way or the other won't cause such dramatic problems.