I find there is considerable latitude in the thinning ratio. Changes or inaccuracy in mixing can often be made up for by changes in air pressure or flow rate. I find a 1:1 mix of MM enamel is a bit too thick for airbrushing, and 1:3 paint:thinner is just a little too thin. Anything between something like 4:5 up to the 1:3 seems to be useable. I sometimes use the 1:3 as a flash coat of gloss on top of several coats at a lower ratio. One has to be careful at the 1:3, though, to prevent runs. Does make a real shiny result, though. I never go that high flats unless I am doing a weathering job, which is at very low flow rate.
Also, I do my brushing at 15 psi- if you use a higher pressure you may want different mixes.