I don't know the exact thinning ratio. I doubt if anybody does, because, without extensive testing, you don't know the viscosity to begin with.
So I thin the dope in a mixing jar and give it a try. If it doesn't go, I add thinner and try again. By now, i can tell if it's ready by the feel of the stirring stick.
Maybe if I added enough thinner I could have sprayed it, but since it's base coat to sanded anyway, I just go to the brush.
The photos are from my first attempt at air brushing when I discovered that "modern" dope won't tolerate brushing over the base coat.