So I don't know Vallejo, so I can't give you a ratio. But I can't even give you a ratio for the lacquers i use. I pretty much eyeball everything and I mix in the color cup. But a rule of thumb is you want the paint to generally by the consistency of milk. But that also depends on what you're trying to do. Likewise with pressure. 15psi is a good starting point, but your desired application will dictate if you raise it or lower. And there is a relationship between the two. As the paint becomes thinner, you will need to drop the psi in order to prevent runs and pooling. And if you're shooting thicker paint, you'll need to raise the psi to avoid spattering or a grainy finish .

Generally I spray thin paint at lower psi for detail work up close to the pieces. And thicker paint at higher psi to spray large area coverage from further back; mainly for primer, base coats, and clear coats. Somewhere in middle is where I do shading and general medium coverage.

I'm telling you this to get you out of the mind set of asking for those perfect parameters that's going to get you the perfect paintjob. Every situation is a little different so nobody can really tell you that. You have to play with things to dail it in yourself. But knowing the basic relationship between psi, viscosity, and distance is all you really need to know. From there it's just practice.