I usually thin both Model Master and Humbrol 40% to 50%. With the Whites and Yellows I will add 5% to 10% more thinner. It's not water thin, perhap somewhere between milk and skim milk (although I find that water thin consistancy with low air pressure and the proper needle adjustment lets me make very thin lines, exhaust stains,oil leaks etc).
To thin I use DuPont Enamel Reducer #3812S. I discovered that it does as good a job as the OEM thinners but is a lot cheaper (my last gallon cost me $13), lasts a lot longer in bulk and does just as good a job.
The only thing I can recommend is to sacrifice a bottle of paint and experiment. I'd start by adding a little more thinner to what I usually use and use the PSI that I normaly use. If that doesn't work, try lowereing the pressure 3 to 5 PSI. Continue to add thinner and vary the pressure until you find what works best.