Other than keeping the acetylene pressure below 15 psi (I don't like going over 10), knowing the pressure isn't so important. You need adequate flow at the tip to keep the flame pushed out and prevent it from jumping inside the tip. While also you need to keep the flow low enough not to push the flame away from the tip. For welding you need to keep the flow low enough not to blow molten metal around. Whatever pressure is takes to do that is whatever it takes. The ratio between acetylene and oxygen you'll set by looking at the flame (specifically the acetylene "feather" part of the flame). Generally this will result in 5 or 6 psi on both. It's difficult to tell the actualy oxygen pressure on my setup since 5 psi doesn't move the gauge a lot. Again, the pressure number isn't really important. The amount of heat, "forcefullness" of the flame, and flame mixture (adjusted visually) are the important things.
For welding I like to adjust the regulator pressures so that I'm running the knobs on the torch almost wide open. When the knobs are almost wide open, you have to make a lot more turns of the knob to get much change in the flame. This lets you easily make fine adjustments. If you have the knobs almost closed, you'll get a flame that really changes with just a small bump of the knob.
For cutting I like to set the acetylene pressures similar to welding, but you need to run the oxygen pressure much higher to get a good "blow out" of the cut material (maybe 30-40 psi). You'll limit the actual oxygen pressure at the tip using the mixture knob. Only the center jet will see the full pressure when you hit the lever.
