I know that there are formulas for switching from and to sourdough and or preferments, but even though my library grows I haven't found such a formula.
I have a general idea of the fact that a sourdough bread usually requires around 40% of sourdough and a regular yeasted bread requires something from .1% to a stunning value of 2% of yeast.
Other that this general idea I have not found anything in the literature I have at home since most of the books usually treat these subjects separately.
Is there a formula that transforms a recipe from fresh yeast to sourdough and vice versa?
What about preferments?
EDIT: The question is under the assumption that a started is intended as a 100% hytration one, if you know of a conversion for a 50% hydration starter or pasta madre feel free to pitch in.
Best Answer
I've seen numbers ranging between 3x to 20x for going between Madre or starter to yeast.
That said, as @Chris H noted, there are multiple other variables and factors involved here that would make such a formula completely moot. Here are some of the factors:
In either case of yeast or starter, what you are trying to do is give an environment to the yeast to digest the sugars and produce CO2 to lift the dough for a given time span.
A starting point:
Have a look at the sour dough's recipe's timeline and try to match that with a yeast based recipe of similar time-line.
If it's a few (8-10) hour rise type of bread, then you'll need a good amount of starter (e.g. 80-125g for 600g flour). See other factors above.
If it's a multi-day yeast based recipe, it'll call for 1/10th or 1/20th of the yeast by comparison (.1% to 2%), but if you go with 1/20th of the starter (e.g. 4g), you may not get much activity in the dough. So you're probably better off erring on the high side in those cases.
make sure you adjust for the hydration present in the starter (count the water in it as a part of the water you need for your hydration percentage)