As Roman and Mr Lu said my normal mix is 20-30% paint and the rest, thinner. Yes, dirty thinner!
And, use the airbrush not as a brush. Let me explain. I think that itīs not Ralph situation, but many modellers as they move the airbrush, they want to see the color on the kit finished. And thatīs a mistake! With the airbrush you have to pass several,many times over the same surface to get your color. You pass one time, then go to another place, to another, then you return again ... and again to another place .... itīs a long and tedious work, but the results are excellent for my taste.
You get not only a smooth surface, even you get a really rich surface because as youīre not a robot and you do not pass the airbrush the same times over all the surfaces. So, as some times itīs a random task, youīll get different plates with different colors due the different number of base coats applied over during the procces. The result is atractive, the same color with different intensities!
If you use a B&W base, of course, as Mr Lu said, the contrast of the base must be extreme!
Anyway, I like that Stug, love the color richness of the surfaces and with a little bit of weathering, the result will be outstanding. Keep on that way!