Create a formula to count unique values?

I am trying to create a report for utilization that is based on the billable percentage of time reported by users for a 40 hr work week (instead of the percentage of billable time based on the actual number of hours worked, as our average # of hrs per employee per week is over 40). I thought about writing a report highlight that would take the number billable hours and divide by the of unique employee IDs that reported time in that week and multiply that times 40, but I couldn't figure out how to have Clarizen count unique values. Is this possible?

Thanks,

Chana