For example I used a 3/8" 4 flute square end mill to cut the flat on the G19 slide for the RMR. My RPM was about 400-500 but I was only taking .005" per pass with a slow feed as well. This is because that slide is diamond hard. I really just listen to the mill and watch the chip load. It will get easier and once you hit the right formula write it down, rinse and repeat.