Jock has a point, but if you want to do this, you need to be a little conversant with baker's percentages, and work backwards from the final dough. You probably are going to need the firm starter to be anywhere from 30 to 90% of the weight of the flour in the dough. And the firm starter is going to be roughly equal amounts of refreshed sourdough starter and flour. Get a copy of Crust and Crumb. Reinhart changed the percentages he uses when he wrote The Apprentice, but I still like the barm in Crumb.