There are many more variables that affect a planet's ability to support advanced life besides it's similarity in mass to Earth. "Simple bit of math based on descent assumptions...." There's your problem. Calculating the probability of all the necessary attributes that must come together for intelligent life to exist is neither "simple" nor should it be based on assumptions. Remember what happens when you assume too much? [ASS-U-ME]. Here's an interesting quote from a prominent astronomer:
Teams of astronomers from all over the world continue to search for strange new worlds. As of December 11, 2009, these groups have found a total of 407 planets. Yet not a single one is an analogue to any of our solar system’s planets. None of the newly discovered planetary systems permit the existence of a planet like Earth. The exuberant vision imparted by Sagan has, for nontheists, turned into a dirge. In his book God: The Failed Hypothesis, atheist and particle physicist Victor Stenger laments that Earth, “a tiny blue speck in a vast universe,” is alone, the only locale where advanced life might exist.
Sorry to burst your bubble, but the Earth and our solar system are unique.