you go with the most accurate entry you can find. in other words if the fast food place has a website, use their info for what you ate. if it's something you can break down part by part, do that and enter everything separately. because when you use a generic entry, the generic entry is either a specific item that you don't necessarily have the recipe for or it's an average of some of the most common recipes. which means it can be really off. for example when i make my version of a broccoli, cheese and rice dish it runs significantly below the generic entries [i use a lot more broccoli and vegetables in it].
-google first. ask questions later.