It’s a tight rope to walk to try and get a sauce to the perfect consistency. All of the following will affect the final thickness of your sauce: temperature of the liquid, type of thickener used, how long the sauce is cooked after thickening, the natural thickeners in the sauce, the type of dairy added at the very end of cooking, whether the sauce is pureed or strained, and how much liquid cooks out of your turkey or chicken. The French have a term called nappe, which is when a sauce coats the back of the spoon in a thin layer. This is what a cook should look for in a good sauce.
Unfortunately, the majority of the thickening power comes from flour, which is added early on in the sauce process. It can prove to be a guessing game as to how much thickener you will need. A trick I like to use is to add extra oil and flour at the beginning of the cooking process. The additional oil and flour will provide extra thickening power at the end of the sauce making process, if you need it. So that I don’t make the sauce too thick, I remove some of the cooked oil/flour mixture and set it aside. If the sauce is too thin at the end of cooking, add some of the reserved oil/flour mixture to your simmering sauce till it’s a perfect nappe.
Our Maple-Cider Brined Turkey with Bourbon-Cider Gravy is a great recipe to try this tip with.