after the FPR is going to tell you what the FPR is letting go by, which isn't the pressure that is actually in the rail. You want the pressure before the FPR.
I think you're thinking backwards in terms of the FPR.
the fuel comes into the rail, through the injectors, then to the FPR. The FPR increases the pressure in the rail by closing up the exit.
make sense?

so the gauge needs to be just before the fuel gets to the injectors to tell you what the fuel pressure is at, at the injectors.