The best advice I can give you is to thin the dope at least 60%, and only brush it out about 3 to 4 inches at a time, and only use two to three strokes without loading up the brush. I load the brush by putting it in the dope about half to three quarters of the way up the bristles, and touching one side of the brush to the inside of the container to keep it from dripping off. You just have to keep the brush moving or it will stick especially when you brush to far without loading the brush up good. It is way better to put on several coats of thin dope than thicker dope, and it will keep brush marks to a minimum. Hope this helps.

I always brush the clear and spray the colors.

In the olden days I brushed everything.  When I tired to do some painting in the 21st century I found that the dope had been reformulated.  When I brush on the second coat the new thinner dissolves the existing color and makes a mess. In the old days this didn't happen.  Dry was dry.  Now I can add thinner to rock-hard dope residue and use it again.

About five years ago I broke down and bought a compressor and a bunch of air brush paraphernalia.  It's the only way to go. 

Taking about thinning by ratio or percentage is pointless.  We don't know how thick the dope was to begin with.  It's an art, based on stirring the painting and thinning by feel.  If it doen't go through the air brush, add more thinner.  I use old glass shot glasses.  When the nozzle clogs because the dope was too thick, clean the nozzle with clean thinner and dump the tinted thinner back into the spray jar.

It's an art, based on stirring the painting and thinning by feel.

Or get a viscosity measuring cup and a stopwatch.