I.E., you can cut Black by a little over half, but Sunburst Yellow can't be more than 1/3.
The amount you cut it by depends on how easy the paint applies. I almost always cut mine with windex, and occasionally with 'airbrush' thinner. I can't really understand how you're having issues with Vallejo's game paint range when thinned (EDIT: Well, it can be rough for the first couple of sessions with some of the more unique colors). I'm pretty new at airbrushing and use my almost exclusively for basecoating, especially the particularly tough colors, like yellow. I can't tell you too much about the appropriate green for necron (I used Scorpion Green) with airbrushes.
