Ok. Double sided test tape would be good test as it removes clamps all together. This should determine if your spindle xy plane is parallel to mdf bed xy plane
I’ve noticed that workpeice can bow if fixing pressure applied too heavy in certain areas. I use more clamps as this helps flatten any bow/warpage issue
Obvious one is making sure your workpeice is flat and uniform thickness. Measure thickness in several places using calipers, place straight edge against edge etc. I machine a lot of acrylic and the thickness can vary +/- 0.15mm across each sheet and cast acrylic is naturally warped / not flat