I love Polly Scale. Here are my recommendations:
Thin to the consistency of 2% milk. That means anything from 10% reducer (thinner to those outside the paint industry ) and up. Pour some 2% milk into a glass and swirl it—note how it looks as it drains back into the bottom—that's what you are looking for. A little too thin, with acrylics, is better than not thin enough.
Get some acrylic retarder from an artist's supply store. About 1 drop in a airbrush cup's worth of paint is all you need. The stuff has to be stirred forever to disperse in the paint, by the way. But it will largely eliminate the tip dry problem.
Thin only with distilled water. Tamiya thinner is alcohol, and Polly Scale is not compatible with alcohol.
I primarily use Tamiya acrylics as primer and for hand painting, because they are thicker. If Polly Scale had basic colors (red, green, blue, yellow, etc.) and was a little thicker, I'd use nothing else. Once you get the knack, you'll see what other's rave about.