I didn't really pay much attention to actual depth( and the maths ;-) other than making sure the bit cuts through the copper fully.
My min trace width setting in the sw(altium) was 15mils.
I use an ancient copy of lpkf software ( from a machine I had years ago ) for the isolation. I wrote an hpgl to gcode translator to process a report that the sw generates.
I'm sure the sw you are using does similar paths.
The rate was about 3mm/sec so ~180mm/min
It could be your copper thickness: it looks like 2oz which might be harder to cut milling-wise(?) Are you in the usa? I can send you a sheet of what I'm using. I got it on ebay way back.
