well it just goes by "feel", theres no ratio that i could come up with because once you open the can and pour some out the next day the paint will be a bit thicker. if i had to guess i would say about 20% thinner or so? just thin enough that it does'nt run, but not too thick. keep in mind that you can allways add more paint to the mix if you go too thin. and the coats go on really really thin, that is the key, like i said before you don't really have a true non transparent base until the 3rd or 4th coat.
There is no way i would try to roll on BC/CC it would definetly not work. that paint is ment to be atomized when sprayed, and you could never thin it down to the point where it does not run and is thin enough to apply. plus it would dry too fast with the correct reducers and flash over too quick. i think it would be impossible. when u just paint and sand, you're going to be sanding too much, and the paint is hard resulting in waves in the paint unless u sand really consistently, so the key is to lay it on thin and sand as little as possible, also when the paint layer is thin and the paint is thinned properly it "self levels" it's like bondo, u mix it up and it's just in a blob, let it sit for a few minutes and it flattens out, thats just what the paint does.
hope that helps