Uneven carving on front/right portion of x-carve

I have noticed the past few projects I’ve tried that whenever I try to carve anything on the front right portion on the x-carve, the depth is off.

almost as if as it is carving, the z axis is rising as the spindle moves closer to the front/right corner causing the depth to be shallower than it is supposed to.

Either uneven wasteboard surface or uneven material thickness.

The first is easiest solved by adding a thinner 2nd layer of wasteboard and perform a “skim cut”. This will make it 100% even relative to Z axis.