OSSA's Troubleshooting Procedures document states that the fuel pressure should be 3.5 ±0.3 kg/cm2 (that's 47 to 54 psi for us metric-impaired Americans). Note that this is to be measured with the engine running. If you are careless enough (as I was) to measure the fuel pressure with a 12-volt battery powering the ECU, it will only produce roughly half that pressure.
Fuel pressure is critical to correct injection volume. Flow is proportional to the square root of the pressure ratio. For example, if you only have 25 psi when you should have 50 psi (25/50 equals a ratio of 0.5) you will only get about 70% of the intended fuel flow.
Excluding external leaks, there are two places the system can lose fuel pressure – the fuel injector itself and/or the fuel pump. If the fuel injector is dirty, it may not close completely (both when it's injecting and when the engine is stopped). A leak in the injector can result in a rich running condition.