Just in case you're still looking for this answer, here's what we've come up with, Amy. You can convert a commercially yeasted dough to a sourdough by making a sourdough build with about 30% of the overall recipe flour. Multiply your total flour weight by 0.30 and add an equal amount of water and then add 10-20% (of the 30% flour weight) of ripe sourdough culture. Allow the mixture to ferment 12-16 hours at room temperature. When you perform your final mix, remember to remove the flour and water weights you used for your sourdough build and to omit the yeast. Because sourdough ferments at a slower rate, you will need to adjust your fermentation times accordingly. Hopefully that helps, and happy baking! 

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