CheersToGreatBeers said:
I understand that the viable cells reduce by approximately 20% each month after the "Mfg Date".
This is where you are mistaken. While this may have been the case 14+ years ago when most of the yeast calculators we use today were developed, the world does not remain static. Like all manufacturers, the yeast companies have made process improvements and lengthened the shelf life, viability, and survival rates of the liquid yeasts. This really throws all the calculators out the window when it comes to predicting the number if viable cells based upon manufacturing dates.
If you look back at my last post, I had a cell count of 109 billion cells with a viability of 98% (rounding down) which would give me 106.8 billion viable cells in the package at 5 weeks old. Projecting this out, at 6 months (26 weeks) the viability would have dropped around 10%. This would equate to 98 billion cells in the package. Given my counting variability of +/- 3 billion cells and assuming that commercial packages are all the same, I would contend that they plan on at least 100 billion cells at the best buy date on the package.
I also would suggest that the use by dates are calculated to the particular strain, which is why you may get different ranges based upon each strain.
The starter calculators are highly variable in their predictions versus your actual results since everyone will make a starter just a little bit differently (volume variations, DME variations, access to O2, time on stir plate, rate of agitation, etc). Unless you are performing yeast cell counts at the time of pitching, you really do not know how many cells you are actually tossing into the fermenter.
Basically, I have gone away from using the viability the yeast starter calculators predict and, since I have the equipment, do my own cell counts when I need to ensure a given pitch rate. My recommendation would be to use the "best if used by" dates as the manufacturing date and assume that you have around 100 billion cells at that point or anytime leading up to that date.