But as Kara explained, such a percentage can be utterly misleading. First of all, deciding how much of a percentage a mission, or a piece of music, or a cursor contributes to the total is damn near impossible. Then there are the parts where you can't plan accurately how much you need of it, like bugtesting, for example. And the public outcry if the percentage ever gets stuck at some value for some time is something best avoided.
For a project like Diaspora, the "we'll tell you if there's something we'd like to tell you"-approach probably works best for all involved.