I am a newbie trying to follow Eckhardt and Butts' recipe for pain au levain.  In step 1 you create a sourdough starter.  In step 2 the starter is used to create a sponge.  In step 3 the sponge is used to create a dough.  The recipe suggests that you set aside a cup of the dough for future breads.  However, I am not clear whether, for the new bread, this leftover cup is the starter for the sponge (step 1) or the sponge for the dough (step 2).  Also, do you use it all or do you use the amount that is indicated for the starter (or for the sponge).