Stage dimensions get messed on resize

programming flash
1

I got this script, which creates a centered rectangle in my movieclip to be used afterwards as the container of a photo gallery interface.

var squaretwo:Sprite = new Sprite();
    squaretwo.graphics.beginFill (0x333333,0.6);
    squaretwo.graphics.drawRect(50,50, stage.stageWidth-100,stage.stageHeight-100);
    squaretwo.graphics.endFill();
    square.addChild(squaretwo);

This works perfect when I launch the thing from the movie in its original state. But If I happen to resize it (particularly if the resized display area has different proportions), the whole thing goes awry. It goes off the center and, depending on the case (I don’t seem to find a consistent rule here that determines the behavior), draws a rectangle thay may be bigger or smaller than the intended size.

Any ideas on how to correct this?

Note: If the square is displayed and I resize the screen there is no problem, but if I close it, resize the screen, and then open a new one, it gets messed up.