I’ve got best results milling the PCB with this kind of tools:

You can find them on ebay with words “Carbide PCB Engraving Bits CNC Router Tool 90 Degree 0.1mm”.

My settings for PCB-GCODE are:

In eagle’s DRC / DRU I usually set the “sizes” tab “minimum width” to 0.8mm, but with carefull positioning of the clad I have achieved 0.4mm minimum widths.