How can I control amount zoom effect in Photoshop's animation timeline?

I've discovered the Timeline in Photoshop CS6. This allows one to create animated presentations of photographs.

I've selected part of a photo, cut it, and placed it on another layer above the base layer. I've then repaired the original base layer to replace the cut pixels using the spot healing brush.

I now want to animate by zooming into the base layer while the top layer remains stationary.

I can do this fine, but I can't find a way to control the amount of zoom — there are no controls for this I can see. How do I do this?