I set the desired parameters and start,at 50% and bump up the feed and monitor spindle speed/load. Also, the is a big difference in tool type/geometry. Typically 3 cu in/min per hp in aluminum. I have a 1/2" zrn serrated rougher from maritool that i can push to 4 cu in/min.These little machines run out of power quick, and it's a fine line between max MRR and stalling the spindle.