I set the fluid knob at 2 1/2 turns out and the fan knob about 1/2 way out.
I then set the regulator on compressor at 90psi @ the regulator on the gun @ to manufacturers specs.
Mine is 29-54 psi.
I copied this from a good place for HVLP setup info.
SETTING UP HVLP GUN
Set the fan width as needed (you don't want to change it after you have "tuned" the gun). Turn out the material knob about 2 1�2 turns. This is the "mixture" adjustment, kind of like the idle screw on a carburetor. The farther in it is screwed the lower the fluid to air ratio is and the smaller the droplets will be. The farther out it is, the higher the fluid to air ratio is and the larger the droplets.
Set the air pressure at the inlet to the gun to the manufacturer's specs. On an HVLP gun, this spec is usually found on the gun, and is the maximum PSI it can have while still maintaining the maximum 10 lb at the cap for legal HVLP transfer efficiency (68 %). You are now ready to do a test sprayout.
Tape a piece of masking paper on the wall for the test. Hold the gun at a right angle to the wall, just as if you were going to paint the wall. Hold the gun at a spread out hands distance (about 8 inches, or 22 centimeters). Pull the trigger to completely open for a split second and then close it. You want an ON-OFF wide open-completely closed in ONE movement. You should have a cigar shaped pattern with complete coverage in the center with fading coverage going away from the full coverage cigar shape in the center. The center should be fully covered without any runs. If you have runs, either you are holding the trigger too long, you are too close, or the gun is simply applying too much material; in which case you need to screw in the material knob or turn the air pressure down. But most likely if you have turned the material knob out the 2 1�2 turns and the air is set at the factory specs, you are just too close or holding the trigger open too long.
The droplets you see trailing off the center are what you will use to "tune" your gun.
Turn in the material knob to make the droplets smaller (and or raise the air pressure). The balance you need to attain is the smallest droplet size possible before you lose the coverage desired. In other words, if you turn in the material knob too far, not enough material will be coming out to cover the panel!
Now, you'll notice that I said, "raise the pressure to the gun", while earlier I said to set it to manufacturers specs. We are talking a very small adjustment. It is a fine balance in material-to-air ratio and a little more air than specified is okay. Even if it is an HVLP gun the inlet pressure recommended is to maintain the 10 lb limit at the cap. Well, about three quarters of the country has no regulations for HVLP use, so if you go over the 10 lbs all it will do is atomize the material a little better. You may lose a little of the benefits of HVLP though. But remember you have a lot of control with the material adjustment knob.
After you are happy with the droplet size, DON'T TOUCH THE FAN CONTROL. It will change the PSI at the cap and will change the atomization you worked hard to get.
AIR REQUIREMENTS FOR HVLP
An HVLP gun requires more VOLUME of air to operate (the V in HVLP, High Volume Low Pressure). Now you may notice that your HVLP gun is adjusted at maybe the same PSI as an old conventional gun, around 50 lbs at the gun (many HVLP guns are set at much lower though) so where is the "Low" in PSI they are talking about? It is at the actual air cap where the air and paint come out. An HVLP gun has only 10 lbs at the cap while a conventional has upwards of 50! So the VOLUME of air (CFM -- Cubic Feet per Minute) is the key to proper atomization with an HVLP.
If you have a gun that requires 15 CFM you will need a compressor and plumbing that will produce that at a very minimum. There are HVLP guns that need as little as 7.5 CFM so you can get good results even from a smaller compressor. Air supply is a complete subject by itself, so let's assume that you have the air supply needed and move on to gun setup.
So atomization is the key, but why? Why can't you just lay it out wet and let it "flow", as an old painter will say. Picture a jar full of bb's; they will represent small, atomized droplets of PSPC. The gaps in between the bb's is solvent. Now picture a jar filled with marbles, they will represent large, poorly atomized droplets of PSPC. The gaps in between are, you guessed it, solvent.
If you apply your PSPC in large poorly atomized droplets, what you will have is a film full of solvent. This can and will cause slow curing, shrinkage and dieback (the loss of gloss in the hours and days after application).
So, now that we have learned the need for gun setup, how do we do it? Let's start with the fluid tip choice. The newer high-solids low-VOC PSPC products need to be broken up more, so a smaller fluid tip is needed.
Basically you want the smallest fluid tip that will still allow you to PSPC the particular part you are PSPC'ing, keeping the entire thing wet and in a fair amount of time. In other words, a 1.0 tip would be beautiful for clearing one fender, but would be lousy to paint a complete. The application would be way too slow, and the first panel would be way too flashed by the time you got back around to it. So you need to compromise -- a 1.3 is a great all-around tip, while a 1.5, though getting a little big, can get you by. If you read the tech sheet on the particular product you are shooting, it will have a recommendation for fluid tip size.
There are needs for other tips. For instance, when shooting polyester primer you may need as big as a 2.3, but for urethanes and epoxies, the 1.3 or 1.4 will work great. If you plan on using a pressure pot or painting a bus, all bets are off and we would need to study a little bit more.
As an example of the use of a 1.3 tip, I did a test once that proved the point well. I shot two panels of metal with a medium solids urethane primer. One was shot with a 1.3 super high atomizing top of the line topcoat gun. The other was shot with a 1.5 (or a 1.7 I can't remember) "hoser" primer gun. Three coats were applied and after a full cure (the one shot with the larger gun took MUCH longer to flash and cure by the way) the film thickness was measured. The one shot with the 1.3 tip was 2 tenths of a mil thicker! The larger gun laid out the marble sized droplets full of solvent, and when the solvent flashed the film shrank.
You need to "tune" your gun EVERY TIME you use it, just as you would tune a guitar before you perform. This is done with a very basic sprayout pattern test. This very basic test tells you how your gun is atomizing and you adjust it to achieve the best atomization you can.