Crap, how did this end up in OBD2? I could have sworn I put this in the general GM EFI systems. All my tuning has been on OBD1 systems, not sure if obd2 if different. Maybe a mod can move it?

I am still confused. I was reading something online about in/hg and it was saying, basically normal ambient air is like 30 in/hg. But all the gauges I see are opposite. Then I see the chart you posted up and the last two rows seem to help. However there are two in/hg columns. One is "absolute" and the other is "gauge", they are complete opposites.
So do you use a different scale when measuring engine vacuum Vs. something like barometric pressure? Is that the difference I am seeing?