The key to "Reductio Ad Absurdum" is that it relies on your final statement to be "absurd" built up from "non-absurd" statements. And of course, with "absurd" being a subjective statement, it relies on that final statement being SO ridiculous, so laughable, that anyone would instantly recognize it as absurd.

Your final statement was not "absurd", merely "improbable". You made several assumptions and then derived a number. However, a number is NOT clearly absurd. It could be imporobable, likely impossible, but not absurd.

A proper "reductio" applied to this issue would have us do the same assumptions you did but we end up with the result that all planets have life (absurd) or that no planets have life (absurd). Anything in-between is not absurd and that is all that you have shown, the in-between.

So no, it isn't obvious at all that "reductio ad absurdum" is what you were trying to do.