Well given how much we have learned about exoplanets, I'd say that at least one number of the assumptions are rather moot. The "how many stars even have planets" can be assumed to be basically "all of them" statistically speaking. ( https://www.nature.com/articles/nature10684 ) and while we can't yet say much about the specifics of their composition (a spectroscopy of the light passing through earth sized exoplanet atmospheres is at the cutting edge of research, with people mostly still working on new instruments, and trying to find lucky candidates where we can get any reading at all with current instruments) but we can definitely impact the lower bounds a bit here since based on current data, about 1 in 5 stars is suspected to have an earth sized planet. ( https://arxiv.org/abs/1311.6806 )

So the "... if only one in a billion of these stars is a sun with a planet... ... and only one in a billion of these is of earth size--" is now " 1 in every 5 stars has an earth sized planet and if only X% are earth like"