I suspect that the cornmeal is for taste and texture, although there really isn't that much of it (one tablespoon for about 4.44 pounds of dough).
I usually use a separate bowl to make the poolish, so I am more likely to experience dough losses as a result of the poolish sticking to the sides of the bowl, wooden spoons, spatulas, etc. In the 14" pizza size example I gave, the weight of the poolish is so small, about 3.5 ounces (I estimate almost a half cup), that it wouldn't make sense to use my mixer bowl. In your case with the full batch size, the poolish weighs about 15.5 ounces, or close to a pound, so it does make sense to use your mixer bowl. But even then you are likely to experience some small dough losses. So, I personally would use a bowl residue compensation of 1.5% when using your method. I compare actual and calculated weights for all of my doughs and it always amazes me how often there are losses, even when I am careful about measuring out the ingredients, scraping the mixer bowl, attachments, my fingers, etc.