I almost always use Vallejo paints, either of the Air, Metallic, Game paints an washes, an the whole line. I just have found them to be perfect for me as far as acryllics go.
That said, I do use some others, including but not limited to Tamiya colours such as the XF-series, which are not airbrush-ready, an I thin my Vallejo paints the same way; trial an error. Usually I start with a 50-50, but in my experience, even different colours, especially metallic paints, are thinner than others.
I have always had a box of cheap plastic spoons on hand to play with ratios. I start with a 50/50 for most paints besides metallic type colours, an add to them when the consistency is about like that of 2% milk. At about or near 18psi on my compressor an airbrush, that is the sweet spot. FOR ME. I've had a friend with exactly the same tools as me crank his compressor to 27psi to get the same result.
The IMPORTANT thing to remember: not all airbrushes, not all compressors are the same, even within their own model numbers. You do NOT have to be spot-on with ratios. What IS important is to thin the end mixture to that of watered-down milk. You can ALWAYS add paint, i.e., add layers. You cannot remove them. At least not easily at all.
Don't get caught up in the numbers. Just thin with appropriate thinner for the type of paint you are using, an test with a scrap piece of spru or plastic spoons. (Spoons are great because you get a good indication on how light will play with the colours because of the convex/converse angles). Spray what is comfortable.
Remember that even too-thin mixes can be added between coverages, but too thick will cause issues with airbrushes an whatever needle you are using. Favor on the side of too thin, an add passes/application of colour or add a drop or two of paint.
Over time, you will get a "feel" of what works an what doesn't. I almost moan when I see posts about "you have to have 13% of this to 27 1/2% of that", because while it might have worked for that person, it will not ever work for another. Work with what you are comfortable with, an remember always that this is supposed to be a fun hobby for YOU, no one else. We are our own worst critics, an I get that, but this is supposed to be a stress-relieving entertaining activity, not a race or show of muscle. It's just like building furniture...measure twice, cut once. Same with thinning. Err on the side of thin.
Good Luck!
Mutt