Hi ReNae, a poolish is traditionally composed of equal parts by weight of flour and water, plus a tiny bit of yeast. Adding equal parts by volume (cups) will give you a much different consistency and the water weight in the poolish will end up being about double the weight of the flour. The percentage of flour used in a preferment can vary a bit, but 30% of total flour sounds fairly typical. If you're attempting to add a poolish to a recipe that doesn't already call for one, I would subtract the water and flour amounts from the total flour and water, rather than adding additional ingredients.  

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