having run spraying projects before and as a model builder that paints with an airbrush in my spare time...
Viscosity is the key.
Find the best viscocity for your equipment and air pressure.
Amount of thinning agent needed will vary from batch to batch of paint so the best check is for viscocity and not a documented ratio. You can buy a viscometer and train people pretty easily, but experienced painters will know by feel.
I've used these in the past, not for paints but it will do:
http://cgi.ebay.com/BROOKFIELD-LVF-...820?pt=LH_DefaultDomain_0&hash=item3cb809b1b4
For my airbrushing my viscocity test is a dab of the thinned paint on a vertical surface. If it runs like whole milk runs, it's good for my airbrush and operating pressure for general painting.