Slight wall of text attack and it may not make much sense, but I've not slept for 36 hours. That's my excuse and I'm sticking to it
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The first thing to take into account is peak fuel pressure at the fuel rail, ideally you do not want to see more than 4.5BAR. Increasing the fuel pressure alters the spray pattern of the injector, alter the spray pattern too much and the fuel will not atomise sufficiently.
What is the purpose of the FPR... why do we need a FPR that has a rising rate of 1:1...
The simple answer is to maintain an equal pressure differencial between the inlet manifold and fuel rail regardless of boost pressure or manifold depression conditions. Base fuel pressure is set via the FPR with the vacuum hose disconnected, this ensures our base fuel pressure is X PSI or X BAR above atmospheric pressure or 0PSIg, this base pressure is the pressure differencial and will always be a constant.
For example; We set our base fuel pressure at 44PSIg (3BAR) (with the engine at idle, vacuum hose disconnected from the FPR), so now our pressure differencial is set at 44PSIg. Next we reconnect the vacuum hose to our FPR and assuming our boost gauge is reading (technically incorrect but used to get the point across) -7.5PSIg or -0.5BAR etc... our fuel pressure should drop by -7.5PSIg from 44PSIg to 36.5PSIg, still maintaining the 44PSIg pressure differential. Likewise if we hit full boost at 14.5PSIg or 1BAR our fuel pressure should increase from 44PSIg to 58.5PSIg, again maintaining the 44PSIg pressure differential.
Why do we need this pressure differential? Think of pressure as the force pushing (it actually pulls but forget that part) the fuel out of the injectors and into the inlet in a similar way to the battery voltage being the force turning the starter motor and the engine. We need 12 volts to make that starter motor turn and we also need our 44PSIg to get the right volume of fuel out of the injector for the time it's switched on. If our battery voltage drops to 6 volts there wont be enough force for the starter motor to do it's job, if our fuel pressure differential drops below 44PSIg then we won't get enough fuel into the engine and risk the chance of 'running lean'.
If we did not use a RRFPR our fuel pressure differential would drop as manifold pressure increased, i.e. if we don't increase fuel pressure by 14.5PSIg when running 14.5PSIg of boost then our pressure differential will drop to 29.5PSIg, this means less fuel flow through the injectors (the ECU only controls how long they're switched on for) and our pistons won't be lasting for very long.
So to summarise a little, take you peak boost pressure and subtract it from 4.5BAR, what you are left with is the maximum pressure differential or base fuel pressure you should run. If the injectors cannot deliver enough fuel with that pressure differential then you need larger injectors.
In extreme cases you can run a peak fuel pressure of more than 4.5BAR, but you need to confirm that the injectors your going to use can work at that peak fuel pressure and also confirm that your fuel pump can flow enough fuel at that fuel pressure. Another factor to consider is fuel temperature, when you compress or pressurise, temperature increase, with an increase in temperature comes a further increase in pressure (as pressure increases, flow decreases), so theoretical guess work is not real life testing. Never run on the limit of your fuel supply system unless you have a spare engine and you like changing them.
