I want to make a tent footprint. I have a piece of tarp and am wondering how much larger than the bottom of the tent to make it. Now I am also wondering if I make it larger than the rainfly, aren't I just making something that is going to collect water under my tent? I see a lot of people making them out of Tyvek, but that also is waterproof, (correct?) and is the same problem as my tarp material. So what's the answer? do I make it smaller than the bottom of the tent, and if so, how much smaller?