I'm not sure what the correct way to approach this would be. I think it may be something like comparing the number of people in your immediate reference class - depending on preference, this could be "yourself precisely" or "everybody who would make or have made the same observation as you" - and then ask "how would nuclear war affect the distribution of such people in that alternate outcome". But that's only if you give each person uniform weighting of course, which has problems of its own.