^
A good thought, one caveat however, works well with severely restricted catalyst not so well with partial failures.
Also the reflective surfaces at the test points must be cleaned, or false readings will be obtained.
(I have seen as much as 325 degF variance by a thin film of dirt,pipe oxidation, etc)

bentley james,
HERE is a great link BradMph posted on vacuum gauge diag.
Download, unzip, click on gauge, all of the gauges are animated with explainations.