Before the transom knee can be attached the final shaping of the transom must be completed. The notch in the transom is 20" wide at the base and 19" from the bottom of the boat, all measures are to the finished edge. All the notch cuts are at a 90° angle to the 15° tilt of the transom. The port and starboard angles are 45° from the top to the bottom.
The instructions provided on this website are not always detailed enough and are "my way" of doing things. Ask 10 different boat builders or woodworkers how to do 1 thing and you'll get 10 different answers.