Here is a little gem, fell out of my cookbook (guess he figured his time was ripe).  It is like a Friendship Bread in that one is given Siegfried sourdough sample and a recipe with instructions.  I think it's funny how it contains both metric and cups.  I would just take a sample of your own sourdough and try it.  How much?  Anywhere from a tablespoon  to 1/2 cup, up to you....  Thoughts & comments welcome.