I would suggest reviewing this: http://www.bushwalking.org.au/FAQ/DIY_RNCTents.htm
This is Roger C.'s web site for his tents. I consider 3 pounds to be a maximum for backpacking for the two of us, 2 would be better. Note that you can also get the plans for two person tarp-tents here: http://www.tarptent.com/projects/tarpdesign.html
A nice looking design, but as Franco pointed out, a more continuous curve is probably needed to prevent puddling. Really, a flat top is not good for shedding rain, but, it could work with proper tension, since it could flow towards the back. Personnaly, I like to simply crawl in, so the back and front are the same size, though.