[MT-2500]

19-20 range is good vacuum.
It is called inches of HG inches of vacuum.
Some gauges read in mm hg and some in g cm but I like it in of vacuum better.
Most vacuum gauges have a green range for good and a red range for bad.
lower vacuum inthe 15-17 range is late timing or a egr valve open or vacuum leak some place.
A flickering vacuum reding is a bad valve amd a engine miss will lower vacuum.

A good cat stopped test can be run with a vacuum gauge to .
check vacuum at idle and then at 2000 rpm.
If vacuum drops at 2000 under idle the exhaust system is restricted.

Also a vacuum gauge on the dash can help get better gas mileage.
Some older cars had one in dash for gas mileage improvement
Keep it in the green 19-20 range for good gas mileage.
I have had it bring gas milage up and over 5 MPG more.
MT