There is no such thing as a cut-and-dried thinning ratio that works all the time. It will vary with pressure, distance from the surface, airbrush construction, painting technique, and the actual bottle of paint in use (older paint that has thickened will need to be thinned more). Try about 3:1 and then vary as necessary from there.
http://www.craigcentral.com/models/thinning.asp