Aha!
The "lower 1/3" as you describe it on the Low Speed side is only used to find Head- and Crosswind components. It's not used when solving wind triangles.
The High Speed side doesn't have the HW/XW section - you need to use the section on the Low Speed side if you want to compute them.
Other than that there is no difference between the use of the two sides except for the speed ranges provided (up to 230 on Low and 150-900 on High?).
Andrew