It really depends on how the system works, most current stuff uses a constant fuel pressure because the injector drivers are more precise and such but back in the day they used a thing called "injector tip delta pressure" which pretty much stayed the same with a vacuum compensated fuel pressure regulator. This allowed to have bigger duty cycles at all the time so the calculations were easier... That pretty much went away when the evaporative emissions requirements pretty much made them put the fuel pressure regulator at the fuel tank to run returnless fuel systems to have less surface area of fuel lines
