jheidt is correct. Using the plumbing code waste fixture unit method would work but it will give you a peak instantaneous flow rather than an average daily flow. Also, in my experience it is very conservative in order to avoid undersizing piping. It would still, I believe, be better to use actual flows from similar facilities.
If you have a mechanical engineer on this job ask him/her if they have already calculated the demand fixture units and the waste fixture units. No need to reinvent this wheel.
good luck