I’ll give you the ATF logic, but do not agree with the ATF policy.
Ares Armor was selling “two tone” resin lower unfinished receivers (80% blanks). The black parts were be left and the colored parts were to be milled out. This showed the citizen who was going to finish the lower receiver EXACTLY what needed to be removed. Apparently, the ATF thought that was too much help to the end user and pushed the receiver beyond the 80%.
Again, their logic, not mine.
I believe they really have no logic. They, ATF, do whatever they feel they can get away with. They just made up that "indentation goes beyond 80%" stuff. But I hear what you're saying.
The Ares guys are wonderful vets and their only "fault" is they believe in the 2nd Amendment.