on doubling a recipe

I often use one of recipes from Bread Alone for Pain Au Levain. it calls for 2 cups of Levain, measured at 18 oz. Unfortunately this recipe only makes 2 loaves. I f I want to bake four or 8 loaves per batch, how do I figure the amount of Levain? Just doubling doesn't seem right, although I have enough Levain to do that. Thanks for a nudge in the right direction. This is a great forum. PG