The Defect Removable Efficiency (DRE) is the percentage of defects that have been removed during an activity, computed with the equation below:
DRE = (Number of Defects Removed / Number of Defects at Start of Process) * 100
We can also calculate DRE as:
DRE = A / (A+B)
where A = Defects by raised by testing team and B = Defects raised by customer
If dre <=0.8 then good product otherwise not.
Also See: some more Software Testing Metrics: