"80%" is, and always has been an inaccurate description. BATFE does not use the term and its also being re-defined all the time.
As for the latest interpretation, from 2008 and directly from BATFE, this appears to be it:
"Here is the new "standard" for 80% AR-style receivers.
"Our Branch has previously determined that dimples or other indexing marks for pivot pin holes are counted as if the holes were fully drilled. Additionally, we have previously found that if any portion of the trigger group cavity area of an AR-type receiver has been milled (regardless of whether or not pivot pin holes are present), then that receiver-blank can be recognized as a "firearm" frame or receiver."
Source: FTB Letter 903050:MMK 311/2008-650