After much research, I learned that a good amount of sourdough starter is about 10 to 20% of the flour weight in fermented flour. … I like easy math. 🙂 Let’s say you use this recipe to make bread: 1000 grams flour (about 10 cups, depending on how you measure flour) 650 grams water (2 3/4 cups) 20 grams salt (4 teaspoons) 2 packages dry yeast And let’s say your sourdough starter is 100% hydration (that is, a 1:1 ratio of flour to water by weight), then, with a 20% fermented flour target in mind, I would use this: 800 grams flour (1000-200, since I want 20% or 200 grams of the flour to be fermented in the sourdough starter) 450 grams water (650-200 because the starter is equal amounts of water and flour) 400 grams sourdough starter (200 grams flour + 200 grams water) 20 grams salt You can use less sourdough starter than 10% fermented flour for sure. … As well, fermented flour has gluten that’s been overdeveloped (gluten develops naturally when flour becomes wet), which isn’t a problem when the fermented flour is used at low amounts, but when a lot of fermented flour is used, it can’t support the dough properly, hence the ugly dimples and inability to rise.