It doesn't have to include these specifics. They are just sub-factors contributing to higher level factors. Your first is covered by "fl, the fraction of those

It doesn't have to include these specifics. They are just sub-factors contributing to higher level factors. Your first is covered by "fl, the fraction of those planets that actually develop life", and your second is covered by "fi, the fraction of planets bearing life on which intelligent, civilized life, has developed". What you are saying seems to be the same as OP. It's not a paradox because the chances are insignificant, so we are indeed alone.