Only way to make someone 100% dead from a nuclear bomb is to put them in the fireball radius. Everything else is uncalculatable variables; Tsutomu Yamaguchi lived another 54 1/2 years after surviving both atomic weapons. It's just not something you can mathematically calculate. If it were possible to account for that many variables and get a realistic number that isn't a complete WAG, we'd never play sports, we'd know the weather forever, and so on.