I commend you for having the courage to use a natural preferment with a Lehmann dough. My experiments doing this taught me a lot and provided some of my best pizza eating experiences.
For an 18-inch Lehmann dough, I would use 20% preferment, by weight of flour. For the recipe you previously used, with 16.10 ounces of flour, the preferment would come to 3.22 ounces. Of course, the nature of the preferment, the hydration ratio of the preferment, etc., will be material, but not insurmountable in the least. It will also matter whether you choose to use cold fermentation (refrigeration) or room-temperature fermentation. You are likely to get a more pronounced crust flavor using a room-temperature fermentation. Fortunately, when I did all my experiments, I reported on them at the Lehmann thread and, hence, have them available to refer to them when I am in the mood to make a naturally-leavened Lehmann dough or to answer questions such as you asked.
As a homework assignment to help you sort things out, that is, whether you should go with a cold fermentation, room-temperature fermentation, use an autolyse, how to adjust the other dough ingredients, etc., you will perhaps want to read Replies ## 151, 161, 165, and 175, starting at page 8 of the Lehmann thread at http://www.pizzamaking.com/forum/index.php/topic,576.140.html
. Reading those posts and you will know as much as I on the subject