Welcome to the forums! You'll find a lot of neat stuff here.
Thinning paint for airbrushing isn't an exact science; even with the same brand of paint, I've found that different bottles often need slightly different ratios for successful spraying.
When I started out airbrushing, a standard rule of thumb was to thin the paint to the consistency of milk. That's worked well for me over the years, regardless of the paint brand. The real secret is to practice with your own airbrush and air supply to get a real "feel" for how it works.
Good luck!
Greg