You may be adding more complexity then is required. A visual verification is usually close enough to get the bike started (and those screws don't need to come out anyway to clean the carbs well). The variations within the cylinders would require additional adjustments to the carbs to be in sync too. If you had no sync tool you could use your method (or similar) to at least get it rideable.
This method is also how most will adjust their FCR's when they convert to a Keihin carb (which the procedures usually say to use a drill bit shank to set the same gaps)