The way to calculate is to first, decide how much starter you want to add to your recipe. Work with a 100% hydration starter. That means your starter is equal parts flour and water. Having the starter as half flour and half water keeps the math simple. Also, most starters are set up with this 50/50 split anyway. I know 100% hydration level sounds strange, but that is considered a baker's percentage for sourdough. Next, calculate the amount of water and flour in your recipe as it is currently written. Once you decide how much starter you want to add, calculate how much flour and how much water is in your starter. Subtract those amounts of flour and water in your starter from the total amounts of flour and water in your target recipe. Example: If you are going to add 100 grams of starter, subtract 50 grams of flour and 50 grams of water from your desired recipe. This should keep the ingredients at the same calculation as in your yeast recipe. You can use this formula with any bread recipe that calls for yeast instead of sourdough, hence turning any commercial yeast recipe into a sourdough bread recipe.

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