Need Advice- Calculation based off of Relationship

[Sammy]

I have a bit of a dilemma: I have a parent database for work orders. One of my fields is a calculation Sum(timesheets_rel::hoursworked) ... This gives me an instant running total when I view a particular record (I really like that part) .. the relationship is based off of the unique serial number of the work orders. I look at individual records every day, and it's useful that it's a unstored, non-lookup field. HOWEVER, I need to run a report on a monthly or bi-monthly basis (or so), and I need to make calculations based off of that Summed value. Any suggestions?