Help adapt formula for use with levain
I've got a formula for a nice American style pizza dough that rises in the refrigerator for 24-48 hours, but I was thinking about swapping the instant dry yeast and long ferment for a levain so I could do silly stuff like make same day sub rolls or maybe even soft dinner rolls. Mostly I just wanted something to experiment with.
The problem is that I just can't seem to grasp how to adapt the formula. I was thinking about plugging say, 20% Biga (100% flour, 60% water, 0.2% yeast) into the following formula in place of the IDY:
AP Flour 100%
I've spent the afternoon playing with the Levain section of Dolf's incredible dough calculator spreadsheet from http://www.thefreshloaf.com/node/4635/dough-calculator-spreadsheet-available  , but I'm still where I started: confused. If anybody could offer some tips about how to do it, I'd be really grateful.
I hope this isn't too obvious a question,