The FMU is a band aid . It's simply another FPR that is mounted downstream from the stock FPR. It adjusts the pressure when boost is applied. A much better solution is to have a tune made for your car which will not only adjust the fuel pressure under boost, but also things like timing and other variables in the EEC. You do not run both a FMU and a tune. Ditch the FMU.
The tune alters/adjusts the injector pulse width to manage the fuel flow whether one is in boost or not. There are literally 100s of variables that can be changed in the ECU but fuel pressure isn't one of them.