Not an answer to your question, but I think the terms "one seat left in the inventory to sell" and "one seat left in positive space on the plane" are equivalent. That is, confirmed reservations that are oversold are just as much "positive space" reservations as the first ticket sold on a flight. I bring this up because although I know what you mean to ask, the term "positive space" has a very specific meaning, and especially if you use it with airline personnel, they're going to interpret it in the standard manner and may miss your meaning.