I don't know if it's possible to give a very accurate roux:liquid ratio because the thickening power of roux decreases as the roux gets darker, and because it can be hard to accurately and repeatably judge the color. Your best bet is to add some of the liquid -- maybe half or so of what you expect to need eventually, whisk until smooth, and bring it to a boil. It should thicken up at that point, and then you can start adding more of the liquid until you get to the consistency you want. Remember that the sauce will thicken a bit more as you cook it due to evaporation, and it also seems to thicken a bit as it cools. So if you start with it just a touch on the thin side, it'll probably be perfect at serving time.
Maybe there does exist a roux color chart that can help, or maybe you're not looking for super accurate numbers. After checking "Cooking" (Peterson), "The New Professional Chef" (Culinary Institute of America), "On Food and Cooking" (McGee), and "Cookwise" (Corriher), the only guideline on ratio I found was from Shirly Corriher, who tells us that you want 1 tablespoon of flour per cup of liquid for a thin sauce, 2 tablespoons per cup for a medium sauce, and 3 tablespoons per cup for a thick sauce. There's a discussion of why darker roux thickens less, of course, but no attempt to quantify that effect. Corriher goes on to point out other variables: age of the flour, protein content (higher protein flour thickens less), and other ingredients (salt, sugar, acid).
Perhaps because there are so many variables that are hard to quantify and control, this is one of those areas where most cooks rely on a little experience and a lot of observation.