Stage.scale question

programming flash
1

I am trying to get the same background scaling effect as yugo site at.

http://amana.jp/company/tsutawaru/

I know how to work with listeners but I’m not quite getting it right. I need to calculate a percentage.

//
Stage.scaleMode = "noScale";
Stage.align = "TL";
//
function centerIt() {
	pic._x = 0;
	pic._y = 0;
	pic._width = Stage.width;
	pic._height = Stage.height;
}
//Size Background and keep it sized correctly
//
var stageListener:Object = new Object();
stageListener.onResize = function() {
	centerIt();
};
Stage.addListener(stageListener);
stageListener.onResize();