This is much more difficult then it appears. You have to also take into consideration if you are using CSF as your RPM will then change and your mm/rev will change depending on the diameter of the part you are at.
It appears that by your numbers that you have rather short feed moves that you are trying to calculate out a time for. If that is the case you can be overshadowed by any rapid moves or position moves with in a given time. These should be taken in to account as well. I would suggest using the machine timers.
Can you not use the machine timers to get what you need? I have spreadsheet that I developed that you can put the rough and finish stock with the SF and feed per rev and it gives you a run time.