What is the purpose of doing such calculations? What do they tell us? You're here. I'm here. Each moment in the world is a blindingly complicated mix of factors. We can't compute all the variables, yet from our knowledge of physics and chemistry, as we look around we can be fairly confident that each moment of the day things are functioning in accordance with their nature. We don't see impossible things happening all around us. It's true (trivially true) that the hypothetical "odds" against things being the way they are today are astronomically high. But so what? Today is obviously not impossible.
So here's the bottom line: long chains of natural causes and consequences happen all the time. In fact, that's what reality is made of. Except for the simplest systems (like the movement of the planets), from any arbitrarily selected starting point (like 10 generations ago) the future cannot be predicted because it's just too complicated. But that doesn't justify anyone in looking back over 10 generations and claiming that it was all an impossible miracle. Thus we have PatrickHenry's law of reality: If each momentary event is natural, the historical totality is natural.
I have discussed this silly business of "calculating the odds" here from time to time, and I've even given it a name: the fallacy of retrospective astonishment. I've also discussed it with logicians and academic philosophers. It seems to be a genuine fallacy. It goes like this:
How could I exist? The odds against it are so amazingly huge!!!
The fallacy involves looking back to some earlier and arbitrarily chosen initial state, then speculating on all the nearly infinite events that might have happened (but which didn't happen), and concluding that the present state has such a low degree of probability that it must have been impossible to achieve by natural means. This "reasoning" makes literally everything impossible (and thus miraculous), and it is therefore an absurdity.