The number the calcs give for new yeast stored in a LHBS's fridge is around that number. I think it is actually .7% per day, but about 20% per month. I've been farming these into jars of approx. 300 billion cells, and I'm down to 1 jar of this particular yeast. So, I wanted to make another big starter and try to approximate it into 300 billion cell jars.
I also just blended this yeast with another so I wanted to keep them isolated in jars so I can mix them at the same ration on brew day.
I didn't think a month would be too big of a deal, and I always use yeast energizer and nutrient in my starters, partially because I store them. With that being said, I also figured the medium the labs store them in are better than mine, and their yeast are healthier. But, I've always read and heard, "if it's a month old, make another starter". But that always applies to brewing with it, not what I'm doing.
I guess, since I'm dividing it by 3, my errors would be reduced anyway. So, even if I estimate it at 300billion at start, and consider it 66% viable, I could assume I'd be starting with 200 billion cells and make my starter as I normally would to get me to that 850-950 billion number I shoot for, then store in 3 jars, I'd be OK.
If I had the money to make the investment I would just start slanting as I have a hard core pressure cooker. But, as they say.. You can wish in one hand and.. nevermind...