In order to get the adapter rail to sit right, as judged by how the contours in the rail on the tail end lined up with the contours bed top rail, I had to push the adapter rail further forward. But this caused the adapter rail to dig into the bed near the cab and push the adapter rail too far...
I have an X4ts, which I assume is very similar. I had an issue with a portion of the bracket hitting the inside of the bed by the cab, which was causing all sorts of gaps and alignment issues. Slightly bending the bracket allowed it to sit better and got rid of the gaps.