Scott,
It depends on the gun. Needle size (orifice) and pressure along with paint feed make a difference. Take a test panel (I usually use an old paint or fuel can) and put all the adjustments in the middle (air valve half way out, paint fan half way between stops, etc). Set the regulator to the recommended (by paint gun manufacturer) pressure. Put paint in and shoot briefly on the test panel. Adjust the air and paint valves until you get a nice, even spray pattern and the paint is well atomized. If you get the the end of the adjustment and are still getting heavy orange peel (glops of paint), the the paint probably needs to be thinned more. After awhile, you get a feel for this based on experience with the gun. I use a little tool I got from the paint shop. You dip it in the paint and let it drip out and it give you an indication of the viscosity. I know what works well in my gun and so, I thin the paint till I hit the mark.