Vertical image panning?

programming flash
1

Hi there, I’m a bit slow with Flash, so forgive me if I’m a complete idiot with this stuff.
Okay, I followed the Interactive Image Panning tutorial here on Kirupa (great place btw) and it works great horizontally, but I’m a bit confused when trying to do it vertically. I changed so far everything from x to y and width to height, but it seems that when I test the movie, and scroll down, the image completely scrolls by, leaving what’s left, which is white. Why won’t it stop at the edge of the image like the horizontal version?

this is what I have at the moment:


this.onMouseMove = function() {
      constrainedMove(bg_mc, 4, 1);
};
function constrainedMove(target:MovieClip, speed:Number, dir:Number) {
    var mousePercent:Number = _ymouse/Stage.height;
    var mSpeed:Number;
    if (dir == 1) {
        mSpeed = mousePercent;
    } else {
        mSpeed = mousePercent;
    }
    target.destY = Math.round(-((target._width-Stage.height)*mSpeed));
    target.onEnterFrame = function() {
        if (target._y == target.destY) {
            delete target.onEnterFrame;
        } else if (target._y>target.destY) {
            target._y -= Math.ceil((target._y-target.destY)*(speed/100));
        } else if (target._y<target.destY) {
            target._y += Math.ceil((target.destY-target._y)*(speed/100));
        }
    };
}