masterKamera,
His primary reasoning with his thinner/flow ratios are to enable consistent fine-line work (1mm paint line) and to prevent tip-dry when using water-based acrylics in an airbrush which you'll get if you don't get the pressures for spraying and paint thinned just right which, as he'll tell you, differs from place to place.
I'll say this: when you order from him, he'll throw in freebies and a handwritten note every time. My sis-in-law, who's been a successful commercial artist for 40+ years and a spook for US Army Spec Ops Command, has started ordering from him and loves the guy, really impressed by his knowledge, professionalism, and products.
I have no connection to this guy other than being a very satisfied customer and as someone that's benefited greatly from his help.
WD