Flatness is the key. Both the machine bed and the board. The gantry doesn't know how flat the board is, so if the board isn't completely flat, your milling depths will be the same, but the end result on the board will be different. That's the biggest thing I have learned on mine. The Shark has a cool feature, called a virtual zero. It makes a virtual flat plane on top of the board surface by measuring the height of the board on all 4 corners. I don't know if other CNC routers have that feature. It comes in more handy for your VCarved work than with pot calls.
Brian