I began by creating a Root 2 Rectangle. First, I drew a 15" square, then measured the diagonal which was 21.25". I took the square root of the diagonal and added that number to the length of one side of the square. The resulting rectangle is 19.6" x 15." (below)
Next, I have to check my work. So I loaded a digital image of my painting onto the computer and changed it to grayscale. When I compare the grayscale image (below left) to the original value study (below right) they match. I don't need to work on the values again.