I generally thin initially to their suggested ratio, then add more if it appears needed. I have generally found that it is not, but I use a gravity fed airbrush, and that makes a difference.
Also, I disagree with Andy: I recommend using only distilled or deionized water to thin Polly Scale. I did some compatibility testing on various paints several years ago, and repeated the process with Polly Scale last year. Polly Scale paints are not compatible with most common alcohols such as isopropyl, ethyl, and methyl (denatured). 70% isopropyl may have enough water in it to act as a "buffering" cosolvent, but I've always had great results with just distilled water. YMMV.