In it he says there are 400 billion or 400,000,000,000 stars, when in reality there are 10^21 stars or 1,000,000,000,000,000,000,000, stars. Wouldn't this make the estimate a lot higher than what he estimated?
Answer on Carl Sagan's "the drake equation" mistake?
the drake equation only talks about the number of civilizations in our galaxy. It is basically trying to predict how many civilizations are within our relative boundaries. Since the nearest galaxy (Andromeda) is 2 million light-years away, it is not conceivable that we would be able to readily communicate with any species from that galaxy within a given time, or any other galaxy that is millions or even billions of light-years from our own.
However, since we know there are about 500 billion to 1 trillion galaxies out there, you can take the equation one step further to calculate the number of civilizations in the universe, which is what you did.