The other thing to note (which you may be factoring in already?) is that the MAP, and hence VCDS, reports absolute air pressure rather than relative. So, when you're thinking in terms of "boost" you need to subtract local barometric pressure... something in the neighborhood of 14.5 psi, depending on altitude and local weather conditions.
IIRC the troubleshooting info in the Bentley does in all in absolute, just to avoid confusion.