I generally start at about half paint, half thinner (more properly, reducer). In spite of what you may hear, the best advice I ever got was: "About the consistency of 2% milk." After measuring with scientific precision for several years, I have come to the conclusion that this advice is, indeed, the best.
However, when you are starting out, I do advise careful measuring. It is easier to keep track of what you are doing, (you really do have a 50/50 mix at the start and not 60/40, for example) and helps you remember what you did.
And don't forget that variations in paint viscosity call for slight adjustments to air pressure and distance.