Finally had some spare time to "prove" the make up of the GM.IBPW pid. What I did was fit the roadrunner, start the car & let it idle. While monitoring the GM.IBPW pid I increased the values in the {B3701} voltage/manvac table while in emulation mode & noted a corresponding increase in the GM.IBPW pid.
So from this we can ascertain that IBPW is contrived from a calculated value plus the offset.
Next I zeroed out the {B4005} short pulse adder table. While once again monitoring GM.IBPW I started adding a value of 1.0ms starting from 1.8ms & working my way up the table. While the GM.IBPW pid was reading 3.1ms at idle, populating the 2.6ms cell in {B4005} caused a surge in the IBPW. Showing that the {B4005} table also runs from the calculated value BEFORE the offset is added.
While I have been stating for a while that this is the case, I am glad I have finally proven to myself that this is correct.
Also note that any transient fuelling addition (wall wetting etc) is also part of the GM.IBPW pid & must be removed if trying to solve backwards for airmass.