The viscosity of the mix of paint vs. thinner is what determines flow-out, runs, sags, brush marks, etc. ... and the viscosity will change due to temp changes and 'humidity' conditions.
A 'viscometer' is the best bet when the conditions are changing to get 'best' - a quite expensive 'instrument'.
The 'flat glass plate' method is a cheap correlation by eyeball to keep the viscosity at the 'correct' value.

For those who 'paint' and want a damn good 'brush job', suggest you go to your local 'automotive paint' supplier and get one of their 'tech manuals' for 'auto body' painting ... and simply apply those 'viscosity control' methods (even the eyeball method) to better control your 'flow out' .... varnish, deck paint, and even 'house paint', etc. Rarely if ever can you take paint straight from a can, dump in a little 'thinner' and have 'visually good' results. reall good paint jobs always are the result of correct 'viscosity' of the paint mix.