Concerning the ratio:
I use a mix that is very very liquid. Thus hardly any paint in a puddle of thinner. Off course only on a future service, which is necessary anyway for applying the decals.
Be carefull choosing the right color!. Your making a Corsair. If I guess you mean the F4 Corsair WWII aircraft, you'll probably make it a deep blue. Using black as a wash, might be too radical. Perhaps you would like a more gentle effect on the panellines, using a darkened version of your overall paint coat.
I've written it several times on the forum now: using a black wash on a white plain, leads to a IMHO cartoonlike result, which I personally do not prefer. Thus, a more gentle effect on the wash is needed. I suggest you browse the forum for built models and study the washes used. From this, discover what is a wash that you prefer.
O, and by-the-by: you shouldn't use a wash on a raised panel line model. Makes a strange effect, and actually makes the panel line seem above the panels.
Best of luck and happy modelling!
Sander