In a good fuel regulator and we assume pump capacity is well enough, it will maintain this 2 bar pressure differential accross the rpm and power band. Meaning at -0.1 bar manifold pressure, the fuel rail pressure will be +1.9bar (approx) and at 0.8 bar manifold pressure.. fuel rail goes up proportionately to 2.0+0.8bar= 2.8 bar ... most cases of the power addict here..boosting 1.8 bar... then it should read 3.8 bar...
The gauge on a AFPR is not reading the manifold pressure, it is reading the fuel rail pressure.
Example, an engine is boosting at 1 bar, and base fuel pressure is 2 bar, that does not mean the gauge on the AFPR will read 3 bar.
Fuel pressure and boost pressure do not increase/decrease proportionately.