hmmm, two bottles, is that 1 oz (both bottles) then or is that the smaller bottles? Not sure but I might try 1 part thinner to 4 parts paint. Again i'm not sure at all just a guess...I could be very wrong, lol.
I usually use about a 60/40 ratio of paint to thinner as a baseline for acrylics, but Frets is right, it's the final consistancy of the mix that matters, you want it about the thickness of milk. Too watery and it won't cover correctly, too thick and it won't spray right, if at all.
Make sure you mix it thoroughly too, those little Badger battery operated paint stirrers do a surprisingly good job and are pretty effortless to use. Some acrylic paints, especially cheap craft paints ,have to be strained as well, an errant blob of semi-solid paint can ruin your basecoat and/or clog your airbrush.