Rolling average number of active employees

CRV10 - I need to write a report showing a rolling average number of active employees for current year.  I'm thinking I  need formulas for nbr active first day of month, nbr active last day of month and average nbr for the month.  Then somehow I need to have a rolling average.  So if Jan there were 100 active employees the avg would be 100.  If Feb there were 75 active employees on the last day, then the rolling average would be 100 + 75 divided by 2 = 87.5 and so on.  I'm thinking the end result would look something like this:
Avg Emps:  end of Jan    end of Feb    end of Mar   end of Apr ......
It needs to be current year only.  My fields are {EMPLOYEE_DATE_HIRED} and {EMPLOYEE_DATE_TERMED}.
The report will be auto run and emailed monthly so I can't use parameters.  I would appreciate your recommendations.