In a two-stage bread, the starter is one column and the final dough is a second column, there's a third 'total' column as well.
If you refresh your starter like this: 500 grams of starter, 500 grams of flour, 500 grams of water, then in the starter column the flour is 100%, the water is 100% and the total yield for that stage is the amount you would add to the final dough, for example, 120 grams of starter. You'll have to weight it at least once, I guess, since a lot of recipes say to use something like a cup of starter, and the weight of that would vary based on the hydration level of the starter. 'Starter' becomes a row in the table and the amount you add is in the final recipe column.
This tells you that the starter is fed at a 100% hydration level (a common way of feeding a starter). How much starter you use when feeding it is a separate matter, it isn't clear to me exactly how that affects the starter over time. In the Tartine Bread books, Chad Robertson recommends using a very small amount of starter, like 1%, when feeding it, so he'd use 1 gram of starter to 100 grams of flour and 100 grams of water. He calls this an 'immature' starter.
I was recently reading a journal article in which the hydration level of the starter was one of the keys to the microculture the starter develops, higher hydration starters favor a different mix of yeasts and bacteria than lower hydration starters. How often you feed it, what you feed it and what temperature you keep it at are the other components, the article suggested that no matter how you get your starter going (build one, buy one, get one from a friend), those four factors (feeding frequency, hydration, flour, temperature) determine the microculture of the starter over time.
For example, one of the main LABs in sourdough is Fructilactobacillus sanfranciscensis (new nomenclature, lactobacillus was split up into several genuses, with fructilactobacillus being one of them) and in a starter kept at room temperature, fed at least daily, and at 100% or more hydration F. sanfranciscensis will become the dominant LAB, no matter where you live.
I've seen some recipes that have as many as five stages. The intermediate stages, like a starter, soaker, scald or mash, generally get added to the final dough stage, but not always, there's a really complex rye recipe where the stage 1 starter gets added to stage 2, along with other ingredients, then that stage gets added to stage 3, along with more ingredients, then that stage finally gets added to the final dough stage.
The BBGA formatting standard has a place for total % of prefermented flour, I'm not sure what that's used for, but is one of the factors you can manipulate, producing a bread with a different percentage of prefermented flour in it.
I'm playing around with a tool that would allow you to enter your ingredients and the baker's percentage of each (and total yield for each stage) and build an Excel spreadsheet for that recipe so you can adjust the total yield for the recipe and have it recompute each component. (I have plans to go well beyond just a simple spreadsheet generator, but one step at a time!)
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