I don't think about servings at all. I just slurp it all out of a big bowl. I just use about 4 tablespoons of root, and use as much water is needed to soak every drop of good stuff out of it. I know this goes against how most people consume it, but it makes sense to me. I don't like wasting kava making making it too strong, while a lot of the kavalactones can't disperse in the liquid. But I usually knead it at least twice anyway. Obsessive, yes.
But yes I think a shell is 4oz/half cup. The servings will always differ depending on how strong it is, so I don't think there is a standard.