If you subtract the pressure (atmospheric) for whatever elevation you live at does it give you a better number.
It might be giving you an absolute pressure reading - so at sea level adding 1 bar, 14.7 psi or 100 kPa.(all approximate).
I can take a look in my notes (in Jeep) or a reading tomorrow - its dark out and there are blacksnakes playing in our yard at night so I'm not keen to stumble out there in t'dark but someone else may be able to give you an idea of the numbers of the top of their heads.
As I remember:
My boost gauge reads 0 psi at idle (whereas UltraGauge) reads absolute at idle of 14.7 psi
Boost gauge reads 20 psi @ WOT (close it could go a bit harder)
Whereas @ near WOT Ultragauge reads 20 psi (roughly) +14.7 psi (1 atm)= 34.7 psi = 239 kPa
It would probably go to 24 psi absolute which is 266 KPa absolute.
I know it isn't a direct answer to your question - I need time for that.
Hope this helps for now.
Mongo have too much to remember for present.