The air supply in the booth should be at 75psi, the paint gun uses a needle valve not a regulator to restrict the flow down to what ever the manufacture recommends 21-35 psi for instance. It is not recommeded to put a mini diaphragm regulator at the paint gun, Follow your gun manufactures instructions. HVLP and PR guns typicaly use the needle air metering valves. You will not get proper air flow through an HVLP when using the mini regulator at the gun. I have often seen guys using them on their HVLP paint guns when I go out and visit them and wondered why the paint gun sounded like it was starving for air, I do know from experience they work better with out them. I believe it's a CFM related issue. When using an HVLP it should make some noise. The first thing I do when watching a guy demonstrate his painting set up is listen to the sound of the paint gun. It needs to go PSHHHHHHH !!! - Not - ppssss..... If you have a mini diaphram regulator on your gun and you only get a 4-5" fan width when set to the manufactures recommeded pressure then try turning up the pressure.
Good information on air system set up