Rune,
Have a look at the thread END MILL SPEEDS AND FEEDS in Members Doc's. It provides basic information and suggested rates for machining.
A program calculates based on some generic info ( actualy over time studies have been done to arrive at the info basis). Thus if your system can't do what the calculated basis was ( ie; not many spindles turn at 24,000 rpm) then you need to modify the feedrate for the spindle speed you will be using. Additionaly, machining experience certainly can be the deciding factor. ie; If your machine has backlash,you can snap
a small end mill in a heartbeat.
RICH