Sorry, still labouring my point I know, but boost indicator does NOT show absolute pressure. Not in the strict engineering sense.
Maybe not in your strict engineering sense, but it did back in the day read absolute pressure, in as much you just added 14.7 to the figure displayed. From
https://www.flightglobal.com/pdfarch...0-%202839.html authored by JOSEPH LOWREY, B.Sc.(Eng.)
A boost pressure gauge, in spite of its name, differs fundamentally from the fuel pressure gauges, oil pressure gauges, and other pressure-measuring instruments used in an aircraft. Like an altimeter, it records absolute pressure, and not merely pressure above atmospheric. In fact, the American name of "manifold pressure gauge" is more correct, the gauge giving no indication of the extent to which the supercharger has boosted atmospheric pressure. The orthodox British system of boost gauge calibration ignores this fundamental fact, pressures being given in pounds per square inch above or below an absolute pressure of 14.7 pounds per square inch. The latter pressure, which is the mean sea level atmospheric pressure, is arbitrarily known as "zero boost."