Z axis not equals

hi all
i am new in the cnc world
i have an issue which is
the z axis in left side and the right side are equal, but in the middle its change
any suggestion !?


MDF is not dimensionally stable, it will shrink/expand with varying temperature/humidity levels.
It is also not mounted parallell to you X-gantry.

The solution is “skimming your waste board”
Many add a second layer of MDF on top of original surface board and skim that.