First you need to make a cutout for the record label. On my platter the cutout is 1mm deep. My present platter is made to be used with outer ring clamp so (other than the label cutout) my platter is flat. When a record is clamped by center clamp only, it becomes concave and only touches the platter close to the label, while the rest of the record is lifted up just a tiny fraction of a mm. Platters slightly concave towards the middle have better contact with a clamped record. I would make the platter surface go down 0.15mm from outer edge to the edge of the label cutout. I took the number from the top of my head so you might want to find another source of info on that.
Marek
Marek