Thinning and pressure?

Thru practice and experience,although there are some general rules,different paints and airbrushes have different characteristics which are learned rhru practice.Thats why many have practice models or use plastic spoons.

You seemed to have found the right combo thru your experimenting.

Each airbrush also adds its own variables to the airbrushing/thinning/air pressure equation, so what combination works well with one, may not work as well with another. Also as Tony mentioned above, certain colors have their own quirks as well. Yellows and Whites tend to require more adjustment efforts than say Blues, Reds, Grays, etc. It likely has something to do with the pigments if all else is the same. Keep at it and you'll find your happy place in time.

Added to the above is also needle size.

Speaking for my own situation though, given what I have for airbrushes and the ones I use for enamel they turn on from about 25psi upward. Everything behaves better there, flow, spray pattern, leveling just everything. And that's whether thinned 60/40 paint to thinner or 50/50. I save 15-20 psi for lacquer. If I spray washes ( rare) then even 10 psi.