Zooming with a pinch motion

[Glyphs]

Hi all,

I have created a stack allowing the user to zoom on an image, and it was actually easy with the "How do I implement a multi-touch pinch motion?" lesson. Now, I need my images to have a min size of 320*480 when zoomed in, and a max size when zoomed out. Does someone know how I could implement these additional functionalities?