Totaly agree with Ivan. Not every paint may have the same thinning needs, even within the same brand. Some colors may need more thinning than others, for example, glossy colors and silky colors usually need more thinning. I found the best way for brush painting is to soak the brush in thinner and then inmediately soak it in the paint and test in a scrap paper, if to thin, then I'll soak it again in paint, and again until I find the best density. There is no really a science on that.