I would suggest using depth instead of parallaxX. By using depth values you can adjust the speeds (with INFINITE meaning no scrolling), and it ensures things stay looking good without having strange differences in scroll speeds. In general, it's far easy to think about "how far away is this layer?" than to think about "what proportion of the camera scrolling speed should this layer have?". The depth field was designed to make adding background layers easier, so you don't have to manually adjust parallax scrolling speeds.