As Jack mentioned, I have used a vacuum gauge to set idle mixtures for years... It can leave you lean if you don't look at what you are doing.. Basically, I look for the point where the high vacuum mark starts to be realized (while moving the screw from rich to lean) and I leave the setting on the rich side of the "high vacuum zone" on the screw...
There is also one other, highly "unscientific" method called the "Dick" tune method (after my good friend Dick Wilson) for setting idle mixture screws on Holley carbs. Taking a close look at either a tach or a vacuum gauge, slowly turn the mixture screw in (lean) until the engine rpm or vacuum JUST begins to fall off. Look at your mixture screw and back it out exactly 1/2 turn.. THAT's IT! This method can be done "on the road" without a chest full of tools and guages and ironically, it usually works out to be VERY close to the proper mixture setting for a Holley.... So, just "DICK" tune it.. as we used to say...
