Ive only been airbrushing Tamiya paints for a very short time. Before that, I used almost exclusively Model Master enamels.
Ive been using 91% Isopropyl Alcohol to thin my Tamiya paints. Some people agree w/ this, and others vehemently disagree w/ it. So far Ive had good results w/ it, so until I have otherwise, thats what I'll continue to use.
I started by mixing them 50/50 w/ the Alcohol, and had very inconsistent results. Sometimes would have great results, other times had nothing but trouble.
A very good friend of mine is the owner of the local hobby shop. I was talking to him a few weeks ago about the problems I was having w/ painting my current project (Tamiya Ducati 916). I was trying to airbrush a couple metallic colors, and it just wasnt going well at all. Spitting, very rough texture, etc. I had been thinning it 50% w/ the 91% Iso. He told me, that w/ acrylics, that you want to thin them as little as possible, and still have it thin enough to go through the airbrush. He suggested 70/30 or 75/25, paint to thinner. I went back, and thinned it about 70/30, wasnt extremely precise on it, and it sprayed a lot better.
So it really all comes down to trial & error, like mentioned before.