My rough calculations showed 697 as the minimum defense rating, but it was based on an estimate of the ratio scaling from 70 to 80 at approximately 2.1. 689 would put it at more like 2.076, which is closer to my original estimate of 2.08, but was inconsistent with some of the other data I saw from the screenshots on MMO Champion. It's close enough, though.
Edit: It is also no longer necessary to hit 102.4 avoidance, since Crushing Blows will no longer occur from raid-level bosses. This should relax some of the itemization restrictions and allow tanks to gear for additional shield block value.