[totalspoon]

I'm always confused by the old imperial versus continental metric boost ratings. I like mm of mercury, a far better system than trying to convert one to the other.

How to convert boost..
The German measured their boost in atmospheres from a vacuum. A DB605 without MW50 was limited to 1.42 Atmospheres (ATA) or .42 atmospheres above normal air pressure. Any reading below 1 ATA means fuel is being sucked in by vacuum from the cylinders and not forced in by the supercharger. The absolute minimum is zero.

The English measured boost in pounds per square inch above 1 atmosphere. There is 15 PSI to an atmosphere. Because readings are taken from 1 Atmosphere, you can have negative readings on an English blower gauge i.e. -7.5 PSI indicates that the boost is half an atmosphere (7.5/15) and fuel is being sucked in by vacuum from the cylinders and not forced in by the supercharger. The absolute minimum is -15PSI
So a Merlin 60 series running on 150 grade was capable of 25 PSI which mean 40 PSI from vacuum (25 + 15) and equates to 2.67 ATA (40/15)

The Americans measured their boost in inches of mercury from vacuum. There