Does anyone know how to use aggregate/summary fields in calculat io

I am try to write a report builder report that calculates the overtime hours
each day for all hours over 8, I have an if statement that is calculating
the daily OT and it works. My problem is trying to do a if statement using
the employee summary aggregate total hours field. I have an aggregate field
that totals the employees weekly hours and am trying to use it in an if
statement to calculate the over 40 but the if statement always comes up with
zero, any idea? Any help would be greatly appreciated!