3 axis Surface milling in WF4

I am surface milling the top of part and set the offset increment to my desired axial depth of cut, the only way I can get multiple step depths is by entering the number of cuts. The issue with this is that I am cutting air in some areas of the surface. I have assembled a workpiece of the blank that I am using, does anyone know how I can select this so the surface milling knows the material that I need to remove.

Thanks,

Rich