My way is anything but scientific, no real formula or fixed ratios, but it works for me. Here goes.
The expression "thinned to the consistency of 2% milk" seems reasonable to me, but what does that look like exactly? I've poured some in a glass jar, dipped the stirring stick in and placed a drop on the inside of the jar. That gave me the consistency of the milk drop as it slid down the side of the jar, but I'll be darned if it seems the same to me with thinned paint.
I think what works for me is to thin to less than the consistency of 2% milk, in some cases with Tamiya acrylic I have made it ridiculously thin, maybe as much as 1:3, paint to thinner. Might sound weird, but it really went on well.
For enamel I begin with a 3:2 paint to thinner ratio, but almost always have to make adjustments as I go through the test spraying process, before actually beginning the project spraying work. For me it was a lengthy learn as I went process, but what I do now seems intuitive and pretty much easy.
I'd suggest maybe buying a batch of paint, start playing with thinning ratios and just spray away. I imagine we all have variations from normally recommended formulas, but I consider them only base line suggestions, likely we end up just doing what works well for us.
You might check out the publications available here at FSM, I think Aaron and others have written plenty of guides for painting.
Best of luck with your efforts.
Patrick