Actually, this is very true. It is like arithmetic, because remember what the point of all of this is: we're dividing up the estate in fee simple absolute that the grantor starts with (usually, at least in all our examples) amongs successive owners/possessors, creating both present and future interests. So, you should always ask: 1) how much of the estate has the grantor given away (all of it? some? If he hasn't given all of it away, he has to retain some kind of reversionary future interest); 2) if he hasn't given it all away, what has he given away, and what has he kept?; and 3) of what he's given away, has he split that at all, and if so, how has he split it (has he created remainders following a life estate? What kind of remainders are they? If they're vested, has he given someone else an executory interest?).