Another STL question

A different problem with STL carve.
I want to carve a topo map with a position base of Z 6mm, and when I go to the detail carve, the carve wants to sink the bit down to the 6mm base depth, I do not want this, I want the carve to run out at the top of the material so that I can shape a bowl using easel, it looks silly with an 1/8th inch wide trough around the item. There is a lake in the center of the bowl and the trough will let the water run out allover my shop floor
Been struggling with this for a few months now.

Got a pic?

My machine is down right now but when I get the parts from 1f and I get caught up will try one again. Wondering if I could just do the rough carve again with a ball nose bit & a very small step over. The rough carve does not do the perimeter gouge,
tomorrow will check the scrap bin, but in the cold weather we have been burning mistakes in the fireplace.