camera pan adjusts BG layer's x-position accordingly

Hi all,
Suppose you don't have the ability to buy Red Giants Horizon, a plugin that helps you create seemless panoramic BG's, so that whatever move your camera makes, the sky 'follows' in a natural way, so that it looks real (instead of having to have a 3d BG layer that's 1000000000x1000000000 px)
And suppose you want your camera to pan from left to right, and you want your background layer to move accordingly but in the opposite direction, so that it looks sort of real.

I'm looking for an expression that can do this. And I'm thinking the amount of movement for the BG layer needs to be divided by something, because it is 'further away'.

I'm pretty new to expressions, so I can't really figure out how to do this yet. Pickwhipping the BG layers POS to the camera's X-point of interest divided by something doesn't seem to work: I lose sight of of the BG layer...

To keep it simple: I'm only looking to change the x-pos of my BG layer when I pan my camera (x-axis). BUT in the opposite direction, AND divided by a certain factor.

Thanks a million in advance!
Marije