Using vacuum gauge to tune engine?

Start by warming up the engine fully. Get it to idle best you can and closest to target idle speed, by adjusting mixture and idle speed screws. Then carefully adjust timing for maximum smooth idle vacuum. "Temper" this with timing light readings so you don't get down the wrong road. This setting may be temporary, depending on your advance curve, IE you may need to re-work that. Temporarily disconnect and plug vacuum advance if used. On a side note I dislike full manifold vacuum for advance

Now readjust idle speed and mixture. Set mixture alternately for maximum vacuum, readjusting idle speed if needed. "Finish' by touching up idle mixture towards lean just a touch. Vacuum should not drop or just barely start to drop at that point.

Now see what you have.

If you suspect port to manifold leaks don't forget to check around UNDER the manifold as well!!! And ports can leak from within the valley if mismatched enough. At least on a B/RB you can examine the valley gasket to port match on the heads.