I use only Vallejo's thinner, and ratios really depend on the pigment level, I find. A darker color can take a lot more thinning than a light one. For example, if I'm going to thin white, I will use much less thinner than if I'm going to thin a dark brown. I try not to thin the paint in the cup beyond 1/3 thinner to 2/3 paint. I usually shoot paints around 20 psi, I work from a bit of a distance, and I spray light and with quick motions. If I am working up close, I work around 10 psi or less, again spraying very light and with quick motions.
I do find I have to pay attention to the air brush nozzle tip, it dries out very easily. I use an Aztek, but I don't think the drying problem would change with any other brush.
Other than that, I've been playing with them for a couple of years now, and I guess have finally gotten comfortable with all their little idiosyncrasies.
Thanks for the comment.