the fuel pressure in the injector rail should be set with the engine running at idle speed with the pressure connection to the fuel pressure regulator disconnected and blocked (the pipe).

the pressure should be set to 3.5 bar as standard.

the fuel pressure regulator adjust the pressure according to the signal from the pressure pipe. so at idle when you have vacuum there, the pressure will drop to around 3 bar, at 0 bar (open throttle, no boost) it will be 3.5 bar, at 0.5 bar boost it will be 4 bar, 1 bar boost = 4.5 bar fuel pressure etc.

however, some chips are specifically written to use a higher fuel pressure than standard. you should check with the writer of the chip

hope that helps my iberian friends