Prophetical means like a prophet, or to do with what a prophet says or does — the role, the speech, the books. It points at the person and the work, not only at a guess that later came true. Though actually, older English treated prophetic and prophetical almost as twins, and some careful or church-y writers still use both. In school English today, prophetic is the normal word. Prophetical can sound old-fashioned, bookish, or religious. If you are not sure which to pick, prophetic will almost always be the one people expect. You are not being more correct by choosing the longer form; you are only choosing the rarer coat.