Meatboy,

Assuming your starter is 50/50 water flour (by weight), you would use 30.71g of your activated starter. It contains the 15.36 g of water and 15.36 g of flour that shows up in the "Total Formula". In this case the "Preferment" would be you starter.

If you where to use a larger amount of "Preferment", say 20% of the total dough weight, then it might make sense to use a bit of your starter (say a tablespoon) to make a "leaven" by adding the bit of starter to a larger amount of equal weights of flour and water and letting this "leaven" become active. The "leaven" would the be used as the "Preferment" in the dough calculator.

If you play with the numbers in the dough calculator, you will get a better sense of how it works. Just remember that the "Preferment" + "Final Dough" = "Total Formula"

Edit - Sorry Bill I posted this at the wrong time.