Yes, these masses should be correct. In the explanatory video you can stop at 0:44 (
https://www.youtube.com/watch). This middle part can be enlarged and reduced in width and then fixed with screws. If you push it completely into each other, it could come to 1100mm. I have a 1600mm plate and the frame is also much smaller (less wide) than the holes would have allowed, as I have mounted the PC outside the feet at the bottom of the plate.