Hi, there's no exact answer to the thinners question, there are many factors to consider. Firstly different colours of paint will have slightly different viscosities & some will 'thin out' quicker that others. Different air brushes will perform better with different thickness of paint, air temperature & air pressure at the gun will all have effects on the performance, as will the size of the piece thats being painted.
That said there are some ball park figures to start with. Personally when using humbrol I start with 60-40 paint to thinners, but always try out on a test piece & experiment with various amounts of thinners. The more you use it , the more you get the feel for it.
The most useful tip I was ever given is, clean the air brush out throughly, & then clean it out thoroughly again. The tinyest piece of dirt will have a major effect on the airbrush & its much easier to clean wet paint out than it is to clean dry paint out.
Cheers Glenn