If the cylinders are severely out of whack (> 3" Hg difference), the ATF will continue to rise and eventually flow into one carb - quickly pull off the tubes and shut down the engine. Then it pays to crudely synchronize using a vacuum gauge on each cylinder before using the manometer.
You can synchronize 2 carbs at a time with the differential manometer. The other two nipples should be capped.
Turn the engine off to move from one pair of nipples to
another. If you don't, when you remove one tube, the lovely red ATF
will get sucked into the other carb.
In my case, it's 3/16" tubing stuck into the top of a bicycle water bottle, with a little hole drilled into the top. To start gas flow, you cover the hole with a finger, squeeze the bottle 'til gas fills the tube, then release the hole. To stop gas flow, lower the bottle below the carbs.
The good thing about the 3/16" tubing is that it fits over the Concours vacuum nipples and press-fits into the 5/16" gas line to the carbs.
I know this works, it's extremely accurate, and it's frugal, which rests well with my Scottish blood.