You create the interface in a graphics program and measure the coords there using available tools (rulers, rectangular selections etc). As far as I remember they are relative to top left corner of the parent element, I'm only not sure anymore if it's x1, y1, x2, y2 (coords of opposite corners) or x, y, width, height (coords of one corner + size). And yea, it's not particularly fun work. Maybe there is some easier way to do it but I don't know it.
