The controlsHeight property defines the height on the background texture, for the controls bars.
In other words this means, that the coordinate is not in Widget space, but in texture space. If your background image has a height of 1000 and you size the Widget to a height of 250, a value of 500 means 500 * 250 / 1000 = 125 Pixel on the HUD.
This makes sure, that the controls bars are the same size on the background image no matter what size your Widget has (the background image will always be scaled to the same size as the Widget (without border).