Okay, I checked it out.
Mine is a '91 300SE, so should be same setup.
At warm idle in neutral, approx 750 rpm, the vacuum gauge is
almost pegged, less than 1/10 inch from the peg, maybe 0.050".
If I increase rpm just a tad, say 850, the needle slides over and hits the peg.
As I said, mine runs about 16 on the mighty vac, so I can assume that the gauge pegs about 17 " mercury?

Cheers,
DG