If you have a depth problem that varies across the machine, my first thought would be that your wasteboard is not level. If you haven’t flattened your wasteboard, you may want to consider buying a wasteboard cutter and knocking down the high spots. That will also let you see if your spindle is truly square to the wasteboard, which is important if you are going to be doing a lot of deep cuts or pockets.
I have made a video showing how I made a new wasteboard, and it includes information about leveling and checking square. You can find it in this thread: X-Carve Maintenance/Troubleshooting Videos - Add Your Own! - #8 by RobertA_Rieke.