In which workflow did you insert the code to do the calculation?
I'm not an expert at workflows, but it seems to me that if you put the code in Bug_MoveTo, then it should update when you just highlight the defect in the list of defects.
I'm not an expert at workflows, but it seems to me that if you put the code in Bug_MoveTo, then it should update when you just highlight the defect in the list of defects.