There is a step you should do before using a manometer.
If you suspect the balance is way off, you should remove the carbs to the bench. Once on the bench, you can use a strip of paper as a feeler gauge. Slide the feeler gauge under each butterfly, and adjust the centre (sync) screw for equal drag on the feeler gauge. That will get the carbs very closely sychronized.
This step prevents sucking the fluid into one of the cylinders.
I have found no need to use any restriction in the lines. The levels wil pulse somewhat, but just adjust until the levels are approximately equal and you're done.
Be aware that only a very small adjustment will make a big difference in the levels. Its a very sensitive gauge, and that is why its very accurate. Vacuum gauges are meant to measure intake vacuum, not differential pressure. They are not very accurate to balance carbs.