Ron - you are exactly right. The weaving requires a slow pace, and it is relaxing as you get into a rhythm.
There's roughly 180ft of cord per lb, and the backrest required 3.6lbs, so right near 650ft. The seat area is about the same width as the backrest, but it's deeper than the backrest's height. However, on the seat, only the top surface is woven, so I'm guessing it will require about the same amount of cord as the backrest.