What's a good cornstarch/flour to broth ratio for a chicken-broth gravy?
For gravy, i think it's better to start with the roux and add the hot liquid to it until you get the consistency that you want, instead of the other way around.  

To make the roux, after you make a roast, leave a couple of tablespoons of fat in the pan (or just melt some butter) and add an equal amount of flour and cook for a couple of minutes to get rid of the starchy taste.  Then add the hot liquid a ladle-full at a time until it's the right thickness.