They did something like this after the Iraq war, where they claimed several million were killed. How did they arrive at that number? They estimated, at pre-war levels of population increase, how many people there ought to be. Then estimated (*estimated*) how many people there were in fact. The delta was supposedly the number Bush killed.
All of it was mush statistics, and assumed people continue to have babies after a calamity at the same rates they did before, which is preposterous.