The remaining growth factors are based on an empirical study done by Chris White, which is detailed in:
White, Chris, and Jamil Zainasheff. Yeast: The Practical Guide to Beer Fermentation. Boulder, CO: Brewers Publications, 2010. 139-44. Print.
This model is based on an inoculation rate leading to a yeast growth factor. We approximated results from that study and fitted it to an equation provided below. There are some caveats to the model to be aware of.
The maximum growth factor is 6 (the starter will never grow past that amount).
The saturation point for a starter is 200 million cells/ml (the starter not grow at that inoculation rate).
The growth curve is geared for a starter gravity of 1.036 (9 ° P).
The initial study did not address stir plates or shaking methods. However, anecdotally, aeration and stirring are said to provide positive benefits. We do not have a citation for this unfortunately, but if you can point us towards one we would be happy to update this page and the calculator logic.