It sounds like you're not understanding exactly how a fuel pressure regulator works. Please forgive me if you do, and I'm not understanding your question correctly. But here I go.
When you start the car, the fuel pressure regulator, of course, controls fuel pressure in the fuel rail. In simplest terms, the regulator keeps the pressure difference between the fuel pressure in the rail and the air pressure in the manifold constant. It does this by applying the intake manifold pressure (whether it's vacuum or boost) to the regulator, which then mechnically "translates" that into the correct fuel pressure.
Let's start with a stock, non-adjustable fuel pressure regulator. When the car is not running, there is no vacuum or boost pressure in the intake manifold. The fuel pressure regulator (assuming the key is on and the fuel pump is running) will adjust fuel pressure to the so-called "base setting". If I remember, for a stock regulator, that's about 39 psi. So, there will be a 39 psi pressure difference between the fuel pressure and the air pressure in the manifold.
Now you start your car and warm it up. At idle, there will be vacuum in the intake manifold - let's say 15 inches of mercury, which converts to 7.4 psi OF VACUUM (i.e. "negative pressure"). So, you will see that your fuel pressure (with the vacuum line still connected) has DROPPED by about 7.4 psi, down to 31.6 (approximately, no fuel pressure regulator is perfect). Again, if you do the math, you will see that the pressure difference between the fuel pressure in the rail and the pressure in the intake manifold is still 39 psi. 31.6 psi in the fuel rail minus negative 7.4 psi in the intake manifold equals 39 psi.
Now, the light turns green, and you stomp on the gas. Let's say your turbo is set to 20 psi of boost. That 20 psi of boost gets sent to the fuel pressure regulator via the vacuum line. The fuel pressure regulator responds by moving fuel pressure UP to 59 psi (if everything is working correctly). This is what people mean by "there should be a one-for-one increase in fuel pressure with boost". If your boost is 20 psi, your fuel pressure in the rail should be 20 psi above the base setting, which in this example is 59 psi. And, as before, the difference in pressure between the fuel rail and intake manifold is 59 psi (rail) minus 20 psi (intake manifold) = 39 psi - the base setting.
The reason a fuel pressure regulator does this is simple - it keeps the amount of fuel being delivered by the fuel injectors independent of the pressure or vacuum in the intake manifold. So, if your fuel injector is firing for 10 milliseconds (ms), then the amount of fuel being delivered is the same regardless of whether you are under boost or under vacuum, because there is always the same amount of pressure difference between the fuel rail and intake manifold. This makes designing the ECU programming a helk of a lot easier!
For an adjustable fuel pressure regulator, there is one important difference - you can modify that "base setting". So, instead of 39 psi, you can change it to 43 psi, or 45 psi, or whatever. This is always adjusted with the vacuum hose DISCONNECTED, because the fuel pressure regulator goes to its "base setting" when no vacuum or boost is applied to it. Just remember, if you increase fuel pressure, the injectors will deliver more fuel AT ALL OPERATING CONDTIONS - vacuum, boost, or whatever. So, if you increase fuel pressure and make no other changes, all of your BLM's will go down over time, because the ECU will sense that more fuel is being delivered. It will respond by lowering BLM's and firing the injectors for a lower amount of time at all operating conditions.
Clear as mud?
