Are you going to run this with a regulator, or totally closed-loop without a regulator? If closed loop, it seems like anything below would cause a target pressure below regulator pressure would just mean nothing goes through the return. If set above, then the duty cycle will climb to 100% because it never reaches target. So open loop seems the way to go with a regulator, that way you can tune in to always provide enough duty cycle to keep the pressure just above regulator pressure.
I was considering this route, but my current plans are to put my flex fuel sensor on the return, in which it would rarely get fuel. However now I'm considering running the flex fuel sensor on the feed line, and PWM'ing it open loop. I'll be running this on an E85 LS1 swap with 630cc EV14's.
My expectation is that there will be X%DC which is the minimum duty cycle required to maintain adequate fuel pressure, and there will be Y%DC which is the duty cycle that starts to overpower the factory regulator. My expectation is that there will be a pretty wide range of "correct" duty cycles which will give adequate pressure without blowing the FPR open. Not an issue for you, but I see this as a great option for people who need a big pump but don't necessarily need the adjustability of an aftermarket FPR.
It should change in lockstep with actual injector PW. I expect the "Y" number to hit 100% pretty early on, too - way before max boost, probably even before you get into boost at all.
It will change with PW (well, PW + RPM, so duty cycle) and injector size, it will also change with a ton of other things. Model of the pump, voltage at the pump, health of the pump, temperature, pressure in the tank, manifold pressure, resistance to flow in the rest of the fuel system, etc. Hell, it probably varies depending on the temperature and viscosity of the fuel.
Looking at the data I took before I installed a regulator, with the stock NB regulator and a Walbro on my 99 it took 4600 RPM and 220 kpa MAP (at a DC of 40% on my id1000s) to bring the fuel pressure down to the nominal regulator set point.