There are 359 Tory MPs according to Parliament's clerks. One of these is under investigation by the police for alleged sexual abuse and has been asked by the Whips not to attend the Commons. So it is not clear whether today's electorate is 359 or 358. I will assume it is 359.
Now, as I said earlier, Tory MPs think that if the PM wins two-thirds of the vote tonight, he would be wounded but would have no hesitation in carrying on. But it is worth pointing out that a vote against him by a third would mean 120 Tory MPs had lost confidence in him. They would be equivalent to 62% of all backbench MPs, or 62% of those MPs neither on Boris Johnson's ministerial payroll or in some other way dependent on his patronage. If the outcome were worse for Johnson, say 40% were to oppose him, then 145 would have lost faith in him. And that would represent 75% of all Tory MPs not dependent on his patronage.
What do these numbers mean? Well if Johnson were only to win by 55% to 45%, it would mean almost immediate curtains for him, based on the precedent of the leadership election of 1990 (under a different system), in which the most successful Tory leader of the modern age, Thatcher, won by a decisive margin of ten percentage points over her opponent and abstainers, but quit within two days.
So Johnson has to do better than 55%. But how much better? Well at the end of 2019, Theresa May survived a confidence vote by winning just under two-thirds of the vote, but in the succeeding weeks those alienated MPs only became more discontented and their numbers increased, so six months later she resigned.
The implication is that for Boris Johnson to be confident he has not been fatally wounded by tonight's vote, he probably needs to win support from considerably more than two-thirds of his MPs. It is a high hurdle for him to leap, especially after his own anti-corruption tsar John Penrose has quit saying that Sue Gray's report into parties proved Boris Johnson was no longer fit to be PM.