That question can open a bag of worms. It depends on your gun - siphon,hvlp,lvlp,gravity, brand name as well as the paint viscosity(thickness). My Iwata likes 9-16 at the inlet, my Sharpe, 23-25, my Devillbiss, 25-28 and all are hvlp. If you have a Harbor Freight purple gun, 45 to 55 is about right - and your neighbors will hate you for the overspray covering their tomatos. You need to shoot a test pattern and a test panel. The pattern to make sure you have a Bill Clintonish cigar, the panel to prove wetness/coverage. Start with the owners manual for the gun and go from there. Then there is the air quality - will your compressor maintain even flow and pressure - and is it dry air.