There are some variables, pressure, airbrush needle size, ambient temp and number of coats desired. But an initial ratio as you stated, (1:2) sounds right in the ball park.
For me, a pressure of 15-18 psi is usual, thinning anywhere from thinner to paint ratio of 1:3, to as much as 1:1 and medium needle size. I keep my hobby bench spray booth area at 70 degrees.
Really, (I think,) for me the only practical means of determining reduction ratio is to use good light, sample sprays and seeing what is working or what isn't. That's been my method for many years, regardless of type or brand.
I'd be interested in knowing what your test results may develope for a method, if you're inclined to share.
Best
Patrick