"Sorry for not having advice in inches." HA! That's quite alright, I was looking for general advise. Which you gave, thanks. I was thinking about cutting it close but then I had another idea.
I'm going to do like you said and cut a 6in outline around my pad/bag but double the material. So when it's folded in half it will be the footprint you describe but 2 layers. If it rains cats and dogs, I can get under one of the layers (I'm not a fan of bivys). This may be some good insurance against heavy rain with low weight cost, like a polycro burito.
Bob, I also dig your bathtub floor idea. I could probabely do that with packing tape on the polycro.
Anyway, a new material to try out, and reduced pack weight, should be fun.