I have never understood the factor involving the Rate of star formation. Its unclear to me how this is related to the final result, and I think its wrong for one simple reason. If the current rate of star formation happens to be zero, then the final answer to N is also zero.

So in other words, with billions of currently existing stars in the galaxy, but none being currently formed, there are QED zero other civilizations with which communication is possible. Someone, please explain.