I've said before, 12 points is the figure for me.
Not that it 'has' to be 12 points, a much smaller lead can be perfectly adequate & usually is, but any side who gets 4 wins behind a genuine 'top' team,playing well, has shit it unless something really bizarre happens to both. 12 points, at any stage, usually means curtains. That goes all the way back to the 70s when the equivalent 8 points behind a Leeds or a Liverpool meant your season was over.
12 points gap means that averaged out, you have to lose a game less than the 'top' side, in every quarter, whilst also matching them for wins. Then the key bit; having busted your balls doing that & the other team having a blip from hell, you have to not slip up yourself, at the end.