Here are a few of the rules I've tried....
StaffHours.WorkHours=SUM Job.Hours WHERE (StaffHours.Staff=Job.Hours)
StaffHours.WorkHours=SUM Job.Hours WHERE (Job.Hours=StaffHours.Staff)
StaffHours.WorkHours=SUM Job.Hours WHERE (StaffHours.Staff IN Job.Hours)
StaffHours.WorkHours=SUM Job.Hours WHERE (Job.Hours IN StaffHours.Staff)
StaffHours.WorkHours=SUM Job.Hours WHERE (StaffHours.Staff IN Staff.Job)
StaffHours.WorkHours=SUM Job.Hours WHERE (StaffHours.Staff IN Job.Staff)
StaffHours.WorkHours=SUM Job.Hours WHERE (Job IN Staff.Job)