a while back i had found that my machine was going deeper as it went along the x (long) axis, not much, but i could notice it. last night i ran a test on something, and i decided to find out what it was. checked the all the rails, all the height, checked the bed, still flat as all get out, checked the sacrificial board, and sure enough, there was a not very noticeable gap between it, and the bed. a few sheetrock screws (perfect for mdf) later, and it was flush with the bed. ran the test again, and everything was the same depth.