How it's calculated

Absolute = atmospheric − vacuum reading 1 inHg = 33.864 mbar % vacuum = (atmospheric − absolute) ÷ atmospheric × 100

Vacuum gauges on pumps and suction cups usually read below atmosphere in inHg (0 to about 29.9) or as negative bar. Pump and process specs are usually in absolute mbar or torr, where 0 is a perfect vacuum.

25 inHg vacuum at sea level ≈ 167 mbar abs ≈ 83.5% vacuum.
Gauge vacuum depends on local atmospheric pressure; absolute does not. At altitude, the same absolute pressure reads as less inHg of vacuum.