Animate position from sourcRectAtTime().height value to 0?

I'm building a .mogrt template that has text boxes which animate on from behind an alpha matte. The text boxes will change size based on how much text is in them. I know there must be a clever way to use expressions to position the boxes upward by exactly their own height and then have them stop exactly at 0? I suspect this would use sourceRectAtTime().height and Key, but can't figure out how to pass this height value to the starting keyframe. Thanks!