I think that estimating fl should take into account the possibility of interstellar panspermia. Life appearing once could be disseminated through the whole galaxy in a few billion years via interstellar comets.
This creates strong observation selection effect: the galaxies where panspermia is possible will create billion times more observers than non-panspermia galaxies, and we are certainly in such a galaxy. So, fl is likely to be 1.
I think that estimating fl should take into account the possibility of interstellar panspermia. Life appearing once could be disseminated through the whole galaxy in a few billion years via interstellar comets.
This creates strong observation selection effect: the galaxies where panspermia is possible will create billion times more observers than non-panspermia galaxies, and we are certainly in such a galaxy. So, fl is likely to be 1.