Unit is correct, of course, but I think this is a good time to bring up the reason for getting an adjustable fuel pressure regulator. exgmguy is also correct that the computer will simply extend the injector pulse width when it sees that the air/fuel ratio is too lean via the oxygen sensors.
Applying vacuum to the FPR will cause the fuel pressure to go down so that there is a constant relative pressure maintained between the fuel rails and the cylinder. If, for example, the intake stroke has 8psi of pull, you want the fuel rail to be pushing at 31psi for a total 39 psi. Replace 39 psi with whatever you set the pressure at with the vacuum line disconnected.
If you set your fuel pressure too low, you can create two problems. One is that the pressure is so low that the injector will trickle instead of spraying the fuel. Think of it like a bottle of Windex. If you pump hard, it has a nice spray that covers your window. If you pull the pump slowly, the fluid will just slowly stream out onto your window. The other problem is that you will be cutting off the amount of fuel available at WOT. Yes, this can lean out your air/fuel ratio at peak horsepower, but it will be abrupt and relatively worthless when it comes to gaining any horsepower because it won't allow for a change that is constant and repeatable. I could go into a lot more detail about why this is so, but for now I will just leave it at something that you just shouldn't do.
The best reason to have an adjustable FPR is that your engine pulls in more air than your injectors can provide fuel, but it's not enough air to require going to the next highest capacity set of fuel injectors. Let's say that you have 24# injectors but you need 28# injectors to meet the fuel requirements for your engine. Just use the following formula:
N = New Fuel Pressure
T = Target Injector Capacity
S = Injector Capacity at 39 psi.
N = ( T / S )^2 * 39
Plugging in the numbers for our example, we get.
53.08 = ( 28 / 24 )^2 * 39
You can set it to 54psi just to be safe.
If you search on Fuel Pressure you'll find many messages where we go into this in a lot more detail, but I figured I'd enter a little refresher.
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351W 89 Mustang GT Convertible