mmHg is indeed reading vacuum. When not making positive manifold pressure (like idling) or during low load cruising, the engine behaves much like a naturally aspirated engine pulling vacuum. Check out http://www.convert-me.com for unit conversions. You will not spool the turbo until there is a pressure difference across the turbine wheel in the hot side. Until you get above atmospheric pressure at the point you're tapping for your boost gauge signal, your gauges shouldn't go above 0, what you'll see at say .95 kg/cm^2 is boost pressure relative to atmospheric.