From the CTTC owners manual circa '87:
"The Turbo Boost Gauge The boost gauge used in the Twin Turbo Corvette is, calibrated in inches of mercury absolute, In effect, the boost gauge reads as a traditional mercury barometer. For example, with the car off, it will read ambient atmospheric pressure, or about 30". At idle it will show about 12-15 inches and at full boost about 50". The gauge is, mounted in the farthest Left-hand air conditioning vent in the center console."
So, in full boost minus atmospheric pressure roughly 30 inches, then every 2 inches (Hg) = 1 pound of pressure (PSI). So, 30Hg would be 0 psi or very close due to atmospheric changes. 40 inches would be roughly 5 psi.
50Hg = 10 psi
52Hg = 11 psi
54Hg = 12 psi
56Hg = 13 psi
60Hg = 15 psi (actually just under 15psi but close enough)
These are not exact correlations but very close, again atmospheric conditions and altitude will have an impact on what the gauge will read and equate to boost. For example: if you were to drive your CTTC at 12,000 feet ( I realize unrealistic ) 10 psi would register as 43-45Hg.
So, my 58Hg here in Texas is not the same as someone who drives in Denver and their gauge reads 58Hg do to altitude and atmospheric difference.
Hope this helps