I've spent the last year switching from lacquer/acrylic paints over to pure acrylic paints, and I've settled on the Vallejo Model Air line, because of health concerns and it's wide availability locally.
I usually include a lengthy discussion of airbrushing this paint in my reviews (search Eric Christianson, including the M1A2 Abrams cover article in IPMS USA two months back), but not always, so I'll summarize my approach here.
I use Vallejo thinner and an inexpensive product called Liquitex Flow Aid to thin their Model Air line (and yes, I feel it does need thinning for the best painting experience). Their 'dropper' type bottles allow precise measuring, which is another reason I like this paint. If you use the same mixing ratio you can pretty much count on the same results.
For a typical painting session, I will use a cheap, disposable clear plastic condiment cup to mix the paint in before pouring the mix into the color cup on my airbrush. My go-to ratio is 2 drops of flow air + 20 drops of thinner + 20 drops of paint. Swish that around and pour it into your airbrush.
I spray at around 20lbs pressure, and I hold the model so that I can see the paint hitting the surface of the model. This is very important with acrylics since you want to lay down ah bunch of very light layers of paint, building up the color on the surface. Keep the model moving constantly so that you don't saturate a single area with paint, causing it to run or drip (which is why I like to actually see the paint hit the surface). In a short time, you will see your color build up on the surface.
I use two airbrushes for all of my work, and the same procedure for both: a cheap old single action Pasche H and a double action Grex 'pistol grip' airbrush. I get very fine lines with both, as well as good overall coverage. I use the Grex when I have a small job since it can spray with just a few drops of paint, and the Pasche for larger jobs. I have found that it is not the airbrush, but the paint, and the user that makes a difference.
As far as adjusting the ratio for smaller and larger jobs, I use a consistent approach:
Small = 1 drop flow aid + 10 drops thinner + 10 drops paint
Larger = 3 drops flow aid + 30 drops thinner + 30 drops paint.
Good luck and remember to have fun!