Flow out also depends on tempeature, wind speed and humidity. At constant temperature, just a little wind has a big effect on drying times. To maintain flow out outside, you hae to constantly change the viscosity as conditions change. Open time for your batch of paint is also a factor. Basically, the right viscosity at 10 AM generally won't be the right viscosity at 11 AM. Using a viscometer of some sort only makes sense if you are painting inside under controlled conditions. A quick empirical measure of flow out (like the glass plate) makes a lot more sense if you are painting outside.