From FreeSpace Wiki
percent-ships-destroyed Boolean function which returns true if the percentage of ships in the listed ships and wings which have been destroyed is greater or equal to the given percentage. For wings, all ships of all waves are used for calculation for the total possible ships to be destroyed. Takes 2 or more arguments: 1: Percentge of destroyed ships at which this function will return true. 2+: List of ships/wing whose destroyed status should be determined.
- Pay special attention to this bit: "For wings, all ships of all waves are used for calculation for the total possible ships to be destroyed."
- If you want to emulate a nonexistent percent-ships-destroyed-delay, you need two events. The first one which has the percent-ships-destroyed SEXP in it, the second with an is-event-true, with the first event specified as the event that must be true.