Good explanation Centaurus!
I suppose that Lightning Mate's explanation of the boost as a relative pressure gauge showing plus or minus relative to current ambient pressure might useful and might be the way it functions in some applications. I have flown several engines with boost pressure gauges (including a Centaurus) and I have never seen one which worked this way. I have only ever seen boost gauges which indicate the pressure in the inlet manifold relative to standard pressure. The boost with the engine stopped will vary according to the atmospheric pressure of the day and one sets static boost (ie the same as the reading with the engine stopped) during the run up to check the reference RMP as described by Brian Abraham.
Don't think the Lightning had boost gauges and I wonder who sets the questions for LM's professional exams! Perhaps he could be specific, which aircraft has a boost gauge which shows zero regardless of the ambient conditions?