I've been doing a slight mod of 3:1 water to paint.
For the airbrush, useable amount;
3 drops of model color.
6 drops of water (distilled).
3 drops of Vallejo thinner.
1 Drop of retarder.
The thinner is more than water BUT the downside is that it is white. Hence lightens the colors. In this ratio though it is OK. Sprays amazingly through my Iwata hpch.
Tip1: Don't buy the Vallejo 17ml thinner bottles. Just buy the artist 524 500ml thinner for a few bucks more and you have enough for a few years!