My MD demonstrated the downfall of unsupported rails - with the Y bowing under the weight of the gantry particularly in the centre of travel. Do you mean you're dropping 1mm from one end of Y to the other?, if you're bold enough it might be worth examining if, from the image of the router, the rails are attached to the end extrusions, whether you can release the bolts on the end extrusion and shimming it up to recover the lost 1mm. Just a thought.