I personally don't see the hype over these polycryo shelters that are being made. They dont breath at all, not durable, cant withstand high winds...
Why not just buy 3 yards of silnylon for ~$15 and make a tarp from that. Will last you a long time, much better in wind, and overall way more durable.
If you do want to try your luck with the polycryo tarp, I think there is an article in the MYOG section of BPL talking about different pitches/sizes to use when making a polycryo tarp.