So now you double click on the wallpaper to see the overlay, which just shows an appropriately sized green square. Tap inside the square to move the cube around, and the horizontal and vertical indicators appear to help guide you:
And you can resize the cube directly by dragging on the edges of the square.
Works very nicely indeed. All that remains is to make this quick settings screen look nice when in Landscape mode, and also to account for the slight offset in the cube position when not on the centre home screen, I need to line up the square and guidelines correctly.