Fuel pressure and fuel volume are two different things. The FPR addresses both when placed on the fuel rail. Think of your proposition like a garden hose. Pressure just behind your spray nozzle (or fuel injector) is greater when the nozzle is closed than when it's opened. It also varied with how wide the sprayer is open. The pressure at the sprayer isn't regulated and, because the potential volume is regulated upstream of the sprayer, pressure is variable. Think of how you can spray water further when you first squeeze the nozzle as compared to letting it run and then aiming for your target, such as that far off flower bed.

You could get fancy and determine the fuel demand at all possible engine speeds and throttle positions and calibrate a regulator to allow the exact amount of fuel through necessary to maintain the proper pressure, however. I think a rising rate FPR takes this sort of approach, using manifold vacuum as a signal, but I'm not very familiar with those.
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Justin 66 122E
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