I am not sure that the math will be all that difficult if you are able to use the preferment dough calculating tool at http://www.pizzamaking.com/preferment_calculator.html
. I see the issue being more one of logistics. To start, you should be able to use your standard Lehmann total dough formulation but exclude the commercial yeast (more on this below). Next, you will have to decide what kind of natural preferment you want to use with that formulation. For example, will it be a "liquid" preferment or a "sponge" preferment as discussed in Ed Wood's book Classic Sourdoughs
or will it be a natural poolish with a hydration like you are now using? Then you will have to decide what amount of natural preferment to use to replace the commercial yeast (IDY) that you now use with your Lehmann preferment dough formulation. Chau has done some work in this area and may be able to suggest a conversion factor (e.g, "x" ounces of natural preferment is equal to "y" teaspoons of IDY or ADY). Next, you have to decide whether you want to supplement the natural preferment with commercial yeast (this will require including the commercial yeast in the total dough formulation). You will also have to decide on what method you will use to make the dough. That is, will it be a room-temperature fermented dough, a cold fermented dough, or will the dough be subjected to a combination of both room temperature fermentation and cold fermentation. Then you will have to decide the duration of the fermentation process. Somewhere along the way you will have to decide how you will time everything so that you can bake the test pizzas on Tuesday at market. You will also have to decide what dough batch size you want to use for your test pizzas. If it is only enough dough for one pizza, I think you will find that the amount of natural preferment will be very small, perhaps in the teaspoon range.