Bob, this is a very important point you have raised. As you have pointed out, the standard GTiR FPR uses a 1:1 rising rate, which means that at 1 bar of boost the 'rail' fuel pressure should also be 1 bar higher than when the engine is running with no boost. However, the problem is that at idle the engine typically sees vacuum (i.e. negative boost), which means that the actual 'rail' fuel pressure will have more than a 1 bar difference between it and when driving at 1 bar boost. The base GTiR fuel pressure is typically set by:
1 - Remove the vacuum/boost hose going to the FPR.
2 - Measure the fuel pressure.
3 - Adjust to 2.991 bar, or 3.05 kg/cm^2, or 43.4 PSI (depending on your local measurement units!).
4 - Pop back on the FPR hose and drive as normal.
This works great providing your FPR is working in 100% condition. As Bob said above, if its not then you might not be flowing enough fuel (i.e. not having enough fuel pressure) at higher boost values, which will affect your A/F ratios. Remember, that when dealing with fuel pressures, there are actually 2 types of pressure to consider. Injector Dynamics wrote a good article on this, and went as follows (modified slightly to make it applicable to the GTiR):
There are two pressures that people need to consider: rail pressure and effective (or differential) pressure. Rail pressure is self-explanatory; it is the pressure inside the rail. When you stick a fuel pressure sensor on the end of a rail, it is reading the pressure inside of the rail. This is why with a rising-rate FPR you will see the rail pressure increase with increasing boost!
Then we have the 'effective pressure', which is the actual applied pressure to the injector, and is what injector flow rate is ultimately based off of. When an engine is idling, there is a vacuum in the intake manifold. This vacuum essentially pulls fuel out of the injectors, and increases the effective pressure across the injector to a pressure higher than the rail pressure itself. When a supercharged or turbocharged vehicle is in boost, the pressure inside the manifold is trying to push fuel back into injector, resisting the flow and decreases the effective fuel pressure below that of the rail pressure.
This concept is important because it changes how the fuel system needs to be set up. Return style systems will bleed excess fuel back to the tank through the regulator (FPR). Return style systems hold a big advantage in that with a vacuum/boost referenced fuel pressure regulator, the system can maintain a CONSTANT effective fuel pressure, which can extend the range of fuel injectors and help them function at lower fuel demands as well.
With a return system, the base pressure is set with the engine off, but the pump running (i.e. see the steps above). For a GTiR, this pressure is usually set to 43 PSI (factory fuel pressure in the rail). The vacuum/boost referenced regulator will help to change the pressure in the rail based on the pressure in the manifold. When an engine is idling, it may be pulling 20 inHg of vacuum, which translates to roughly 10 PSI. The reference to the regulator will allow it to adjust and lower the pressure in the rail to 33 PSI, resulting in 43 PSI effective pressure, which is the same as the base pressure. When the engine is making 10 PSI boost, the regulator will adjust and increase rail pressure to 53 PSI, again resulting in 43 PSI of effective pressure. The regulator will constantly bleed off pressure inside of the rail to maintain the same effective pressure at all operating conditions. This helps to prevent a loss of effective pressure during wide open throttle, and also helps to prevent injectors from having to run extremely low pulse widths to fuel at idle. A downfall of return systems is the fact that they circulate fuel through a very hot engine bay, ultimately carrying that heat back into your fuel tank.
Some race cars often do away with the rising-rate FPR, and instead stick to a static fuel pressure that matches the desired effective fuel pressure at full boost. This could work something like this:
1 - Remove the vacuum/boost hose going to the FPR (or better still get a non-rising rate static-pressure FPR).
2 - Calculate your maximum boost pressure (say 1.2 bar).
3 - Calculate your maximum desired rail fuel pressure (2.991 + 1.2 = 4.2 bar).
4 - Set the static fuel pressure to 4.2 bar.
5 - Drive car.
This will mean that at low boost and/or idle conditions, the fuel pressure will be far too high (perhaps even something like a 6 bar effective pressure - i.e. remove the 1 bar of boost and add another 1 bar of vacuum!). If the effective fuel pressure is much higher than that at idle you can tailer the fuel map to inject slightly less fuel at these load-low and low-RPM areas by (e.g.) reducing the pulsewidth. However, since the car will be used mostly on track, you'll be running 90+% of the time at full boost and so you'll not have to worry about the FPR being unhealthy and not adjusting fuel pressure properly at high boost levels. Perhaps a better approach if you intend to dedicate your car to track day driving.
Hope this helps,