Unfortunately it's a fairly painful process. From the Track tab, I select Iteration Status and take the total number of User Stories and divide that by the number of defects in that Iteration to gather the defect ratio. Unfortunately just taking the total number of User Stories and Defects isn't an accurate percentage because not every User Story may include QA testing. I am tracking this for multiple teams and have been unable to find any type of report that suits what I'm looking for.