The shiftLightHeight property defines the height on the background texture, for the shift light. The width will be computed according to the image's original aspect ratio.

A value of 0 will make the shift light image be scaled by the same factor as the backgorund image.

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 shift light is always at the same location on the background image no matter what size your Widget has (the background image will aways be scaled to the same size as the Widget.