Agree with Jerry. First get ahold of a Tipi pattern with catenary cut panels.
You can use the catenary chart, buried in BPL somewhere, but nothing beats a pattern.
I've even bought damaged tents just to get patterns. Since the sides of the tipi are often identical, you only need a pattern for one panel. It will save you a lot of time, and avoid disappointing results like you had with the polycro.
For material, please reconsider on the Cuben. It is very difficult to work with. Even some of the professional tentmakers have run into serious problems with it. You don't hear much about that, because no one likes to broadcast failure, especially if they have a business to promote. But word gets around. There are some good forum threads on BPL on making cuben tarps. Read all you can access by searching, and if you can make a tarp, particularly a shaped tarp that you are OK with, then would be the time to move on to the tipi, once you have developed techniques for working with the Cuben. The best MYOG Cuben tarp thread I saw was within the last couple years with a video link that showed his Mum doing the sewing.
A good material for a first tipi, or any other tent for that matter, would be the silnylon sold by Thru-Hiker. It has much higher water resistance than the silnylon generally sold on the market, and less sag because it is more difficult for moisture to penetrate the coat and be absorbed by the nylon, which loves water. But with any silnylon, you'll need buckles or cord locks at the stake-out points to keep the tipi taut.
If you are dead set on using the Cuben, you can always buy whatever quantity you want from Zpacks, or some of the other small companies that get mentioned in these forums, and you can get it shaded so it won't turn your tipi into as much of a hothouse. Even though that costs more, I think it is a better choice than buying direct form CT, because there are a variety of Cubens actually, with different compositions, and you can never be sure what you're getting. With Zpacks, you know the material is coming from a manufacturer who has done his homework and obtained the right material for tent making.