How It Works
The Basic Idea: Sourdough starter is just flour + water + wild yeast. When you add starter to a recipe, you're adding both flour and water \u2014 so you subtract that from what the recipe originally calls for.
Step 1: Decide how much starter to use (as a % of total flour weight). 20% is a good starting point.
Step 2: Calculate the flour and water inside that starter based on its hydration level. At 100% hydration, starter is 50% flour and 50% water by weight.
Step 3: Subtract the starter's flour from the recipe's flour, and the starter's water from the recipe's water.
Timing: Sourdough is slower than commercial yeast. Expect 4-8 hours for bulk fermentation (vs 1-2 hrs with yeast), or do an overnight cold retard in the fridge for best flavour.