Hi F,
I've had the same trouble before, and am now having it again - hence digging up your old post.
After having imported the boujou .cws (with the tracked background) into combustion, I find that reducing the x,y,z scale of the layer you want tracked from 100 down to 1% is enough to sit it in the scene.
I haven't deduced a more scientific way of calculating it yet. Have you?
Ben