Sorry for your headache!! That's what you get for drinking someone else's!!

As I said, I'm a simple guy so I don't try to make things more complicated than necessary. My brews rarely (never ) stay in the primary more than two weeks.
Therefor the difference between floc date vs harvest date is

a few days ... maybe a week. I don't think this time difference is significant. I use harvest date as I believe viability starts decreasing most significantly once the yeast is uncovered from the beer.
The real headache here is that the rate at which the viability declines is significantly variable based on brewer technique, consistency of process, method of storage, and etc. So I don't pretend to predict any success beyond my own. I'm a bit of a stickler with myself on consistent processes and techniques. That way I do the same things wrong the same way every time. This gets baked into my overall process and somewhere in that I compensate for those wrong things and get beer!
The real concept of my reply was to show a simple way to allow BS to do the heavy number lifting brew to brew. Then just accept the answer as a guideline. Without a lab the numbers are all fiction anyway, just like IBU. If you are consistent with your processes and techniques they will guide you toward making your beer the way you want it!
Happy New Year All!
Preston