Having trouble getting elements to hug the top and bottom of the screen on Mobile devices

I have stage.scaleMode to "showAll" and trying to calculate the top using stage.stageHeight after the "added" event, but it doesn't seem to work.

The relevant bits of code look like this:

//default stage is 768x1024
//
screenwidth = stage.stageHeight;
screenheight = stage.stageHeight;

//hide nav above top edge
public function hidenav(){
     navcontainer.y = -400-(screenheight-1024)/2;
}		

//show nav on top edge		
public function shownav(){
     navcontainer.y = -(screenheight-1024)/2;
}