You read Martin correctly, Emmie! To convert the poolish to a sourdough levain, add 1-2 teaspoons ripe sourdough starter (in place of the yeast) to the 166g of flour and 198g of water called for in the poolish, and allow this to rise and ferment overnight. He also recommends adding the yeast called for in the dough portion of the recipe. This will allow you to follow the normal timeline for the recipe. 

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