The best answer I can give is, go to this web site where they have an animated display of gauges (and how they should read with different faults):
Eighteen inches of mercury is healthy vacuum. Yes, some get over twenty, but power boosters, etc., depend on eighteen as a 'standard'.
I like the idea of watching a vacuum gauge as you drive because you can see what happens at the light, when your trans shifts, when you stop, etc. - Dave