" They based them on the observed rate of mutation in successive generations of cells, if I remember correctly."
They couldn't POSSIBLY have been able to calculate all of the variables. They pulled the numbers out of their butts and fooled those who didn't think too hard on what was being *explained*.
Read a textbook on molecular biology (especialy the chapters on error rates of DNA polymerases) and then come back here and discuss this point with some knowledge on your part.