Overtime Calculations...? STUMPED!

Hey you, Guru!

Okay, so here's the rub...

I work in a warehouse-type facility, doing logistics. We have MANY "business units" and MANY "activities." 

An activity describes the task the employee is performing while the business unit describes the client they're performing said activity for. 

What I want is...

..to be able to not only calculate correct overtime costs, but to be able to attribute the overtime hours to a specific activity and business unit. 

Now, by law where I'm at, overtime is defined as any labor hours worked past 40 hours in any work week. Our work weeks are defined by "week ending" periods, Monday through Sunday.

But I can't simply say, "any hours for any employee over 40 for the week ending period is overtime." BECAUSE, for example, say an employee accrued 40 hours doing "Packing" for "Company A," Monday-Thurs. Then on Friday, they worked a normal 8 hour shift still "Packing" but for "Company B." As you can see, we cannot charge Company B overtime because technically, only 8 hours has been worked for them. 

Likewise, we cannot simply say, "any employee over 8 hours for any given day is now in overtime," because they could work 2 x 18 hour days, and still not surpass 40 hours in the week, and therefore not be in overtime. 

However, within our company, we decided that any employee who records more than 8 hours in any day, is doing "overtime labor." BECAUSE management made the decision to keep this person here longer so they could complete whatever it is they were working on before the end of the day. 

So, it's weird. I know I'm going to have to write something along the lines of "If the sum of hours for any employee for any given week ending period is greater than 40, then any time over 8 hours for any day within that same week, is overtime. 

I dunno. This one is tricky AF. Thoughts?