Nor is population density factored in. Urbanization ought to make it easier to kill the first 60% of the world's population. You probably don't need very many

Nor is population density factored in. Urbanization ought to make it easier to kill the first 60% of the world's population. You probably don't need very many bombs to achieve that. Trying to track down and kill every last human being with nuclear weapons is impractical and beside the point. If you kill 60% of the industrial, technologically advanced urban centers of humanity, you have essentially destroyed civilization. Who cares if 40% are still scampering around if they have no energy, no infrastructure, no libraries, no communications, no industry, and agriculture spoilt by nuclear fallout and potentially nuclear winter?