Let's just step back a minute. This isn't about rectangles. This is about the implications of not being able to define sd1 in terms of sd0 without assigning a value to sd0. A limitation which, in my experience, is unique to Pro/E.
The obvious workarounds which can be used to achieve the task given in this example (the rectangle) will not be practical in more complex scenarios.
As a side note, I'm not sure that anyone has grasped that you can define a rectangle by the ratio of the sides and the length of the diagonal. Understanding of that concept is fundamental to understanding this topic.