Human lives deserve whole numbers, not percentages. Ever heard of any "percentage" of people killed in a war?
Two things. One is known as a ground impact burst, which kicks up a lot of the second thing, radioactive fallout. With these two things, megatons are not needed to erase populations, kilotons are enough.
It is the after-effects of a nuclear war that will continue to diminish affected populations through radiation, extremely cold temperatures and starvation.
The talks of Humanity will be over much before a nuke button is pressed, so leave it there only
Let us discuss scenarios only please its better to address how much population of say India or Pakistan can be taken out in direct hits.
Around 3000 nuclear tests has been performed with largest being 50 Megatons of nuke exploded. Not much of nuke winter came our way or starvation.
A 1 MT warhead can only radiates 5 km of area and bring 10 kms of material destruction that is it.