You're missing half the equation. 'Every'[0] national leader is willing to start a nuclear war given some particular set of circumstances. There's always some line that they will not allow their enemies to cross.

The difference between leaders is what those circumstances are. It's in where that line is. [1]

Public communications on this subject have to be taken with a pound of salt. It's in every nuclear nation's interests to claim that line is further out than it really is... As long as they believe that broadcasting the claim itself won't start nuclear war.

It's also in every nuclear nation's interests to be very mindful about where their adversary's line is.

[0] Okay, there might be a closet pacifist somewhere that won't use them in any situation, but if there is, they certainly aren't broadcasting that.

[1] So far, we're batting a thousand for 'No national leader whose last name was not Truman started a nuclear war in a situation where his state was not fighting an existential war'.

This statistic, unfortunately, is not particularly helpful.

It's not measureable. First of you need to account for many having the same weapons (MAD) and you also need to account for the knowledge of the impact of these weapons. The knowledge does change the probability for someone willing and able to use these weapons to come into power (as well as the whole structure around the usage).

Only one national leader -- Harry Truman -- has ever allowed nuclear weapons to be used in combat, in 1945, the same year the weapon became available. This was 77 years ago. He was the 33rd president, and we are on the 46th. By this measure, based only on the US, we get such a leader once every 77 years or once every 13 leaders. These numbers will only get lower once you start to add in other nuclear weapons states.

That said, I don't think any of this math is pointing us toward any kind of answer about how likely any person alive right now is to experience nuclear war, which is really what we're after. At the very most, every day that nobody uses one extends the lower bound of how often they are likely to be used (77 years and counting), but this is almost tautological. Also, maybe it follows some kind of power law and the farther away from their invention, the less likely we are to use them? Or maybe the opposite? Or maybe we oscillate between the two in some sinusoidal pattern? In any case, we can't know. History is not an experimental science.

Which data would that be? We don't know if anyone is willing to use nuclear weapons against a nuclear power because no one has ever done it.

How? We had no nuclear war yet. How can you know that leader is willing to?

If they're not disarming they're either willing to use them or bluffing.