Scratch, I think your calculations are very accurate. The Polycro footprint I use is 100"x 52" and weighs 3.5 oz.s. Add a hood and some velcro tabs and it would weigh just about 4. I draped it over myself and a backpack, and I would use the dimension I have for a poncho. It will also compress to the size of a tennis ball.
I think it would cut, but it does not tear easily by any means. I'm very interested in making some for my wife and I. We carry at least 2 lbs. in rain gear usually, this would reduce it back to a 1/2 lb. I would not trust it on extended periods of rain and high winds on a long trip, but for occasional rain (even hard rains) on a 4-5 day venture I think it would work. Thanks for the great idea. I'm testing some adhesives on it and have found an 8" dia. hole was sufficient for the head opening. I'll post more as I progress, as I hope you will.

One thing I forgot to add: The largest detraction from poncho use for us, was my wife took a bad fall on a rocky downhill when her footing was obscured by the poncho billowing out. The Polycro being clear would allow at least some view of the footing immediately beneath you. I'm also going to make them shorter in the front, as I'm not so interested in keeping my lower legs dry.