At idle.
(keeping it at a constant idle is part of the dance, as your adjustments will directly affect the idle speed)
If you can't do all 4, look at the linkage for how the 4 are connected.
One of the carbs is likely not independently adjustable (linked directly to the main throttle cam) so that one is your reference.
Some carbs are all connected to a common throttle cam, while other use a series of linkages.
(like 2 has an adjustment to 3 and 1, but 4 is slaved to 3 so if you tweak 3 you have also just tweaked 4 as well)
make sense?
Remember, you are not necessarily looking to set to a specific vacuum setting, but rather you are trying to adjust so that each carb is generating the same vacuum relative to the others.
Also, your valve adjustment has a tremendous affect on your synch as well, so make sure that your valves are in spec and that your timing is to spec BEFORE you synch carbs.
Carb synch is usually the last thing you do...
And a 4 way meter system makes things much faster, and since you have to get it up to operating temp and then let it idle while you do it, speed helps prevent overheating. (that and a big ol fan pointed directly down the gullet of the motor while you synch)
