Kill criteria

A usable kill criterion has three parts and fails without any one of them. A specific number — not "if it is not working", which is unfalsifiable, but "fewer than eight paying customers", "revenue below 40,000 in the month", "no repeat purchase from anyone". A specific date, far enough out that the thing has had a fair run and near enough that you have not spent everything by the time it arrives. And a person who knows both, who is not you.

Choosing the number is the hard part, and the test is whether you would actually act on it. If you write a number so low that hitting it would mean total failure, you have not written a criterion, you have written a formality. If you write one so high that you would obviously stop anyway, it does no work. The useful number is the one that makes you slightly uncomfortable — the one where you can imagine reaching it and can also imagine wanting to argue with it.

The third person matters more than either. On the date, you will have a reason why the number does not count: the season was bad, a customer delayed, you were about to change something. Some of those reasons will even be true. The person you named is there to ask whether you would have accepted that reason six months ago when you wrote it down. Choose someone who will actually say it — not the friend who is proud of you.