Yes as Stu rightly pointed out its amazing that many folk dont realise why we use a rising rate fuel pressure regulator!

Put simply any liguid or gas will always flow from a region of high pressure to a region of lower pressure until the pressure is equalised.

If we set our fuel pressure to 4bar at atmosperic pressure then we have an effective 3 bar fuel pressure above atmospheric. For every 1 bar boost we add, we must increase the fuel pressure by the same amount in order to keep the 3 bar differential pressure.

Failure to do this will result in a diminishing injector flow rate as we increase boost!