You want to warm it up good,and watch the gauge as you turn the idle mixture screws..have the idle speed low,like 600 rpm or so,anything higher will give an inaccurate reading because the throttle plates will be open more..
You want to get the highest reading on the gauge,AND the steadiest--if the needle "vibrates" or wiggles,or floats up & down at the highest reading,open the mixture screws a little at a time ,and if it drops an inch or two,but stays steadier,that is where it'll run best...
Then you open the mixture screws just a tad more,so it wont be too lean when the engine is cold..
Unless you have a hotter camshaft than stock,you should get at least 16 to 22 inches of vacuum ,and you can squirt carb cleaner around any suspect areas that may be letting air sneak by like the throttle shaft bushings and base gaskets,and watch for any change in the gauge reading that will pinpoint any vacuum leaks..