Best of three then paper scissors rock.
What’s needed is a clear majority, and of the electorate overall not just those that voted. That in my view should be enough to settle the matter for several decades.
How you achieve that is the real question.
For what it’s worth, I don’t think a second referendum before Brexit would achieve that. Too many people dismiss what they don’t believe as lies so, I suspect, what’s required is material consequences (or the lack of) in daily life.