See the Wikipedia articles on the Torr (a.k.a. "mm Hg") and the barometer (paying particular attention to the "mercury barometer."
Pressure is measured in Atmospheres or Torres. At sea level on Earth you should experience 1 Atmosphere. An Atmosphere is 760 torres so you cut that in half and you'd have a .5 Atmospheres or 380 torres
HI to measure vacuum in hg..that inches of mercury ..thats an old method using a mercury manometer. the simplest andswer is half atmosheric pressure 1000mbar divided by two.