Well.....I definately goofed on my measurements. The zigzag part before felting measured about 13" versus the bottom width of 18". The result is more of an A-line shape. Also, the corners are not where they are supposed to be....so the bottom is funky. It developed little ruffly ridges.
I might cut the bottom section (left and right side) down to size and sew it back together. Or..I'll just enjoy it as it is. I did do a test felt swatch before I started, but I guess it didn't really help me too much on my measurements.
I'll keep experimenting...next time with cheaper yarn