My vacuum at idle on my 270hp dual quad car is a rock steady 18". I have the Duntov 097 cam.
Idle mixture screws are from 1/2 to 3/4 turn out on all four. I am running clone carbs but they are set up just like real deal 270 carbs. Worry less about the number of turns and more about highest vacuum/best idle. When you reach that I always lean the carb idle screws out about 1/16-1/4 turn on each screw. The idle will roughen up then settle down but it prevents over rich idling. I use this tool for the back carbs (I tried a 1/4" box end wrench and those springy flex shaft tools and they suck) - it works but is EXpensive:
You use it withOUT a bit in it and then its a regular 1/4" hex driver. Its well made and includes a ratcheting mechanism and dial indicator so you know EXactly how many turns (or partial turns) you are moving a carb screw...
The incredibly short Philip's head bit for this tool is also perfect for tightening the screw on the back of the turn signal housing using a 1/4" box end wrench to hold it. So - you do get a 'two-for-one' use out of it.
BTW - I run the screws in until the engine acts badly and screw outwards to the highest vacuum point and no farther - so any dead spot is not a factor. I doubt you'll find a YouTube video on these carbs as you are now in an elite cadre of classic car owners with Carter WCFB dual quads - there aren't all that many of us. But the procedure is no different than any other carb.