Here is an interesting Q and A from King Arthur's Baking Sheet (Winter 2010):
QUESTION: From Carl Lindh in Lincoln, California: "Still looking for
information about converting regular bread and roll recipes to sourdough".
ANSWER: Let's do a little exercise to illustrate how you'd make the conversion.
I'm working on some recipes for bread bowls in this issue. Here's one
of the formulas I was thinking about:
3 1/2 cups King Arthur Unbleached All-Purpose Flour (14 3/4 ounces)
1 teaspoon instant yeast
1 cup water (8 ounces)
1 tablespoon non-diastatic malt, (or 2 teaspoons sugar)
1 1/2 teaspoons salt
1 egg white, or 2 teaspoons dried egg white powder mixed with 1
tablespoon water
Manual Method: Mix 1 cup of the flour with the yeast and water, and
let it sit for 1 hour, or until doubled in bulk. Add the malt, salt,
egg white and remaining flour, and mix to form a shaggy mass. Let the
dough rest for 30 minutes, then knead it for 6 to 8 minutes by hand,
or a few minutes less using a mixer. Transfer the dough to a lightly
greased bowls, cover the bowl, and let the dough rise for 2 hours, or
until almost doubled in bulk.
To change this to a sourdough recipe, assume the leavener would
become 1 cup of active sourdough starter. Since most starters are
half water and half flour by weight, to adjust the recipe, you'd
decrease the amount of flour in the recipe by 1 cup (4 1/4 ounces),
and the water by 1/2 cup (4 ounces). The revised recipe ingredients
would then become:
1 cup active starter
2 1/2 cups King Arthur Unbleached All-Purpose Flour (10 5/8 ounces)
1/2 cup water (4 ounces)
1 tablespoon non-diastatic malt (or 2 teaspoons sugar)
1 1/2 teaspoons salt
1 egg white
You could still add the yeast or not, as you like. It depends on how
exuberant your starter happens to be.
Obviously, you'd adjust the consistency of the dough with a little
more flour if you needed to, but I'm hoping this gives you the idea.