On the Today Programme this morning, the political correspondent claimed that this is a Tory leadership election. If Cameron loses, his position as PM will be untenable whereas, if Johnson loses, his ambitions to be PM will be over.
I agree that if Cameron loses he is toast and would have to be replaced by someone from the 'winning' side, hence Boris as massive favourite - so this I think is Boris' calculation - by opting for 'leave' that there is a decent chance that he could be PM by late summer.
If 'Stay' wins, then Cameron is safe, so there won't be a vacancy for perhaps 3 years. That's a long time in politics and enough time for Boris to quietly drop the 'error' of backing the wrong side. Indeed he is already hedging his bets, kind of implying that he wants to vote out, to then renegotiate seriously and ... err ... stay in.
I think one thing is certain - we won't actually see much of Boris in the campaign - he won't be a major player - to do so would damage him too much if his side loses. By contrast he doesn't need to have been a leading player in a successful leave vote for him to be in poll position to take over from Cameron if he is forced to resign having lost the referendum.
He's not daft - very clear political calculation from Boris - but the problem is that it is all too obvious and maybe that is his achilles heal - when there is sufficient passion and the stakes are high I'm not sure the public (or the media) will take too kindly to this sort of overt political game-playing.