Scale Tween MC W/ Dynamic Width & Height

I would like to use the tween class to scale a movie clip. The problem is from what I’ve seen I have to enter a starting scale, and an ending scale for a movie clip.

ex.
new mx.transitions.Tween(redBall, “_xscale”, mx.transitions.easing.Strong.easeOut, 100, 200, 30, false);

Well my movie clip resizes based on the browser so I’m not sure what the actual starting scale value is. I am resizing the image but maintaining the aspect ratio.

What should I do? Can I use the tween class still?