You could do the math and calculate angles, or you could use more direct and intuitive methods.
I use SketchUp a lot, plus a plugin that flattens the 3D model into a pattern. SU also calculates area (and volume), making it easy to figure out footprint space and get a pretty good idea of finished weight.
For these tarps:
I just used pieces of paper cut to 1"=1' scale, and played around until I got something I liked. The design came from a discuss on these forums, and basically follows Jerry Adam's suggestion:
I've also staked out poles with string to get an idea what footprint and head room will be like, and made full-scale mockups with lumber wrap (free from the lumber yard). Some of the lumber wrap I've found has some stretch to it (good for modeling silnylon); other kinds don't stretch much at all, which I expect would be good to see how Cuben would behave.
The tarps in the pic above came in between 8.5 and 9 oz (without guylines etc) in 1.1 oz sil with a finished weight ~1.5 oz. They meet all your criteria, and would obviously weigh significantly less in Cuben--I'd guess easily under 5 oz. I pitch with two hiking poles in inverted "V", which provides plenty of room and is very solid, but have tie-outs for pole-less pitches between trees, etc. Will probably add a zipper to the front beak at some point, as it's a bit of a crawl to get in when the tarp is pitched close to the ground.
Here's another pic, from a different trip:
Apologies if this duplicates stuff you already know or have thought of. I'll check out the calculator you found as well.