I know about injector flowrate and fuel pump and whatever there else is. I don't know why that came up, I never talked about that.
What I'm saying is at 0bar vacuum/boost with fuel pressure at 3 bar the injector injects, lets say 10cc at 2000rpm(just an example). Now with vacuum at say -0.5bar without changing the fuel pressure the engine will suck in more fuel, so the fuelflow into the engine will increase to 10.8cc but to compensate for that the regulator decrease the fuelpressure to 2.5bar so that the fuelflow into the engine remains at 10cc. So 10cc fuel is what the engine would have gotten at 3bar 0bar vacuum/boost although the pressure is not 3 bar, but 2.5bar. That's why I say it's to keep the fuelflow into the engine constant. It's got nothing to do with max injectorflow of fuelpump volume.
I can understand why you see the pressure is 3bar when there is 3.5bar from one side and 0.5bar boost from the other. because the amount of fuel injected into the engine is the same as 3bar although the fuelpressure IS 3.5bar.
It's all got to do with the amount off fuel injected into the engine. That's why I say do not get confused by fuelvolume and fuelpressure. The answer at the end is the same but technically it's wrong.