Anyway, I did the math when this question came up on another board, and using conservative doubling figures you could easily get the population of the UK or even the US.
I'd like to see this math of yours, but it'd best be done in a new topic, I think. If you like. Me, I'm of the opinion that, in the devestated post-flood world, no doubling figure could be conservative enough. You're not just growing a new human population, after all - you're restarting a world's ecology.