Overbuilt yeast starter calculation

[BirchBeer]

BrewUnited Homebrew Dad calculator states that the viability of store bought yeast decreases at a rate of 20%/month. Is this decline in viability the same or similar for an overbuild starter.

Here's the use case:

I have been overbuilding my starter to allow for 2 batches. Due to equipment and time constraints, there is about 2 weeks between my brew days. I do the math and come up with the total cell count that I will need for 2 batches (of different beers using the same yeast) and overbuild for an additional 10% (approx) to allow for decline in viability during the 2 weeks that the second half of the starter is in refrigerated storage.

For example, (using easy numbers) if I need 5B cells for each batch, I'll build the starter to have 10.5B (5B cells for the first brew; and 5B for the second brew (5.5B minus 0.5B decrease in viability for 2 weeks rest in the fridge).

Are my assumptions here correct? Or, do I need to build another starter for the second batch?

 

[DuncB]

Not sure, but only real way to tell would be to work out your cell number at the start of starter production to work accurately in calculator.
With an old yeast the Calculator often reckons you have no cells at all, yet you can build a starter from them.

 

[day_trippr]

I think the OPs methodology looks basically sound, given the stated assumptions about cell viability rate of decline.
I use the same calculator to take the last "over-build" for a given strain and use that to generate enough yeast to pitch 10 gallons of ~70 point wort within the next week plus enough yeast to make it to the next cycle.

It's that last part that's always gray because who know when the next cycle will happen? But in the end, I use the same tool with the elapsed time data to figure out the most likely outcome and cook up that starter accordingly. Seems to work out...

Cheers!