First the theory bit! Atmospheric pressure is taken to be 14.7 psi or 29.9"Hg. This is an average at sea level but is close enough for most of us to work with. A vacuum gauge reads zero at normal atmospheric pressure and if a perfect vacuum was achieved, it would read -29.7"Hg. For our purposes, we can ignore the negative sign as we can safely assume the direction of force is pushing the work onto the chuck