A finished piece should be usable by someone who did not help make it.

Whatever you build carries an invisible layer of knowledge that lives only in your head: why that is named oddly, which button is the real one, what to do when the strange case appears. To you the thing is complete. To anyone else it is complete only if that layer travels with it. Otherwise you have made something that works while you are standing next to it.

Before calling anything done, hand it to a person with no history of it and watch, or write the note they would need. Fix what they stumble on rather than talking them through. Finished means it works without you in the room.