I noticed that the aluminum extrusion/wasteboard was slightly bowed in the front, which caused some issues with cutting too deep and lead to this incident.
I figured out a way to flatten the board without cutting into it. All it took was 2 10mm M5 Screws, 2 M5 Hex Nuts, 2 Washers, 2 Angle brackets, an angle iron, and an 1/8" strip of plywood.
The Final Product - We used an old angle iron that we had. You can see the uneven depth on that waste board I cut into while doing @RobertA_Rieke’s calibration pattern. Also, 2 of my clamps that were damaged in some test projects. I actually bought a 1/2" board of Red Oak from home depot to make new clamps out of.
Now it is almost exactly center. The right edge of the bubble used to be ~1-2mm over the right line.
We put the strip of plywood in to prevent the angle iron from digging into the board or sheering the ends. it distributes the pressure and acts as a shim.
For even more precision I recommend a dial indicator.
Even an inexpensive one from Harbor Freight will do the trick.
Mount the dial indicator to your spindle mount.
This will truly show how flat the machine is, in both X and Y.