It's a given when "solving" a Fermi Problem that your answer is only going to be accurate to a power of ten or so. In other words, if the calculated correct answer to "How many atoms of Jesus do you eat every day?" is 250 trillion, then the actual answer can be expected to be somewhere between 25 trillion and 2.5 quadrillion.
That's not terribly accurate for everyday engineering, of course, but for an educated guess it's pretty good.