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.