I like using the Tamiya thinner to mix my airbrush acrylics.
I use whatever will get me the consistency of milk. Not water or cream.
That is usually about a 60% paint to 40% thinner ratio. But, if I'm going to shoot low airpressure and fine lines, then it's usually a 50/50 ratio.
This is just a starting point to get you going. You'll have to experiment some to see what works good with your airbrush and painting techniques.