IIRC the japanese show boost in mmHg
I'm assuming that is any pressure above or below ambient.
That doesn't really make sense, the A6M5 tops at '30' boost, which by that logic would be 790 mmHg or ~1.03 atm
Most of the German fighters top out in the 1.2-1.4 atm range.
British boost may be in inHg but it says 'LB' on the dial... although I kind of doubt it's psi..