Take a careful look at the fuel diagram. Engine/fuel pump running pressure, the 50 -52 PSI, is determined by the relief valve for the syphon system and depends upon the fuel pump being able to provide enough flow to satisfy the syphon jets. The 40 PSI is the pressure regulator setting which determined the rail pressure a system without a syphon jet but with the syphon jet system it now acts only to hold the 40 PSI on the rail when the fuel fuel pump has shut off. Also I don't think you can assume the fuel pressure regulator on the fuel filter you have in the vehicle has a regulator that is replaceable.

__________________
1988 325is (purchased new) sold
2004 X5 3.0 2005 X3 2.5
2008 X5 3.0 (new to me)