The boostBarPosX property defines the x-coordinate on the background texture, where to draw the boost bar (upper left corner).
In other words this means, that the coordinate is not in Widget space, but in texture space. If your background image has a width of 1000 and you size the Widget to a width of 250, a value of 500 means 500 * 250 / 1000 = 125 Pixel on the HUD.
This makes sure, that the boost bar 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.