I just estimated, cut out too big and adjusted from there. Get one side where you want it then do a templet to match the other. I measured my knee (width) and attempted to simulate how I would be positioned in the boat. Worked well and I am happy with it.

As you can see from the pic, I stripped a large area at the top of the cockpit. I was unsure of actual size at the time I was doing the recess. As for the coaming, you simply glue the vertical strips directy on top of the braces. That came from Nick's added info he sent with the plans. I am glad I went with the braces and will do so on the next.