The only thing I can think of is to plot the co-ordinates of the area to be cut with a plotter, transfer these to your cnc and reduce the height by whatever you want the depth of cut. I did see a plotting machine at an engineering exhibition a few months ago so they are out there, but what they cost, I have no idea. I would imagine the programs will be compatible with G code, however.

You are probably doing this manually at the moment - hence you say, the result are a little imprecise. The ones I saw were accurate to a thousand of an inch, and could be programmed to make as many plots as you wanted within a given area.

Jim Pinder