I got some packers, and I was about to use them like you said on top of the bearing blocks. Meanwhile I searched the net, with a code that was on the ballnut. It turns out it is made by a company called "Vertex Precision". I measured the height from the center of the ballscrew to the top of my ballnut mount and done the same for the supported rail centers to the top of the bearing blocks....strange, they worked out the same. I just had a quick check on the level of the aluminium profiles where they all meet on the X axis, there is a slight difference end to end. I did not build this machine in the first place, and I have already squared up the ballscrew, which was 16mm out from one end to the other over 800mm ! . So hopefully after I strip the profiles down and build it all back up again, everything will sit as it should.