I found that the middle of my wasteboard was about .030" lower than the left and right sides. I originally had a melamine secondary wasteboard on, but ended up replacing it with a thinner MDF secondary wasteboard and used a 3/4" flat bit to cut a level pocket in it.
I made a video on how to do it, if you want something to think about. I also addressed a couple issues in the video, such as how to read the lines left by the large bit to determine if the spindle was truly square to the wasteboard. X-Carve Maintenance/Troubleshooting Videos - Add Your Own! - #8 by RobertA_Rieke