Ah, thanks for the explanation. I interpreted the statement as you trying to demonstrate that number of nuclear winters / number of near misses = 1/100. You are actually asserting this instead, and using the statement to justify ignoring other categories of near misses, since the largest will dominate. That's a completely reasonable approach.

I really wish there was a good way to estimate the accidents per near miss ratio. Maybe medical mistakes? They have drastic consequences if you mess up, but involve a lot of routine paperwork. But this assumes that the dominant factors in the ratio are severity of consequences. (Probably a reasonable assumption. Spikes on steering wheels make better drivers, and bumpers make less careful forklift operators.) I'll look into this when I get a chance.