MANIFOLD PRESSURE: BOOST OR INCHES?q

There is an article in Jan magazine about a U S pilot flying a Spitfire at Duxford. Now some of you modern guys may not be interested, after all it is an old plane and doesn't even have a glass cockpit or Fadec.

But one part brought up a question, how do you convert the British measurement of power, ( lbs. of boost) to the U S measure ( inches of manifold pressure).
The author writes of "inches of boost" which is not right. but I think he means lbs. of boost.
Anyway, I think, but am not certain, that conversion is standard atmoshperic pressure , 30 inches plus double the boost gives the inches as a U S measure.
Therefore normal takeoff of 7 lbs. boost equals 44 inches of manifold pressure.
Likewise a low power of say minus 2 lbs. equals 26 inches.
But I can't find this written, it is not in the Pilot Notes to know for sure?
A max allowed boost of plus 18 would equal 66 inches which corresponds to 66 inch max in a Packard Merlin in a P-51 on normal fuel, so makes sense.

I do wish people that write articles could properly express units...that would make things a lot easier. He's probably speaking of pounds per square inch (psi) and inches of Mercury ("Hg). 1 psi = 2.036 "Hg.