The milk comment always flummoxed me too because whole milk is like a milk shake and skim milk is like water. I read in an article recently that 2% milk is supposed to be the standard.
I use a glass mixing jar and add the amount of paint I think will do the job. I then slowly add the thinner and will stir periodically. After stirring I will tip the jar a bit and watch the paint run back down. Based on how quickly the paint runs back down and the amount of paint that remains on the jar is when I know its ready. This was done over years of trial and error.
I like your approach much better.