I went with the 45 mil (divided by 4). Now don't laugh or shun me but I ended up buying a recycled tarp. I think it's 17 mil. It was 200 and if I love it, I'll redo it with high end EPDM liner. If I hate it, I can rip it out and not feel terrible. If it lasts 3-5 years I'll be tickled.