Bread-Bakers v111.n009.4

Step 1: calculate the total amount of flour and the total amount of 
water (both by weight) in the original recipe's completed dough. 
Total means the starter + the additional ingredients.

Step 2: choose which fraction of the total flour you want to 
preferment, say 20%

Step 3: Calculate the amount of water to use in your preferment 
(which will depend on how wet you want your preferment; I use the 
same weight of water as flour

Step 4: Calculate how much instant yeast for the new formula--since 
you'll be missing the leavening power of the starter. A sandwich 
bread usually has 0.8% instant yeast by weight...or about 2 
tsp./pound of flour. Artisan breads usually use about half that and 
are let to bulk ferment & rise twice as long.

Step 5: Make your preferment--wait 12-15 hours

Step 6: Mix up a batch of dough using that amount of preferment, the 
total amount of flour - the flour in the preferment, the total amount 
of water - the water in the preferment, and the calculated amount of yeast.

Allen
SHB
San Francisco