Happy to help.
"Pre-thinned for airbrush use" is rather meaningless. Generally it means only that the paint in question can be pushed through an airbrush. But for optimal results, you will want to thin it further for yourself. Since Steve packs his paints in plastic containers, the volatiles in it are outgassing though the bottles as soon as they are filled. So these missing solvents must be replaced by adding suitable reducer. Thinning by ratio is a poor practice (especially as we know that Zero Paints have almost by definition varying solvent levels), but I would start with 1/1 or even 2/1 thinner/paint.
I
expect that that's the thinner intended to go with Steve's clearcoat- although as I haven't used his clearcoat I can't say for certain (as I'm on this side of the pond, I use PPG's clear and reducer). If there was a good way of reaching Steve
I'd ask him, but failing that the cellulose thinner is my bet for thinning his clearcoat.