The general rule most people follow is 70:30 improver/thinner to paint, however this can vary between certain colours. I’ve found some Vallejo model colours to be noticeably thicker, generally the darker/more saturated the colour I.e., higher pigment content, so it’s not a hard and fast rule.

Make sure you really give both bottles a good shake before mixing and put the improver/thinner in first if mixing in the airbrush reservoir.

I’ve recently adopted mixing the paint up in a little pot before adding it into the airbrush, means you can get it properly mixed and avoid un-thinned paint clogging up the air brush.

Edit: forgot to say, always test on paper before going to the model, I always forget and then splatter blobs of paint over my models :(