When I made my first tarp, I wish I had these plans. I now have them for my second.
One thing that I did and will do on the next is to make the cat cut, I used a piece of flexable rod, (like a tent pole), place the rod long the bottom edge, then push the middle up to the depth on the cut, mark and cut. A few different weights to help hold the of in place. The first one, I started to measure and mark, using the math, it was a PITA, got a few feet in, looked at the design, looked at the rest of the numbers, and bam, it is just a curve, why not find an easier method. Works like the post with the thin piece for wood and string.