Slightly higher idle will bring the vacuum up slightly, also, as well as timing.
I don't know the specs on your cam, but iirc with my HKS 264's, I'd get 16-17", but I'm close to sea level (you probably are too), fwiw, and I want to say my idle is *slightly* higher than stock. Rather than trusting a boost gauge, use a diagnostic vacuum gauge instead, as it's more accurate; if you're that concerned. More important that strictly the reading is how does it behave; as in is it steady, or does it dip and go back up, dip, repeat, etc...
here's a good diagram