You probably already tried adjusting the feed plane to .1 or so above top of stock. Your acrually wanting to cut. And use depth of cuts. I would use surface rough parrell and select the surfaces you want to machine. But select the tall wall as a checked surface does that make any scence? Edit looking at part in cam I see what you mean now. I would still try the surface rough and select only the surfaces or high speed path rough with a raster pass.