Good question - 5e
The reason I picked 8 ft was to see if the corner was the only issue when I originally set the distance for 5 feet but that wasn't the case. It was really just the top row in its entirety. The other surprising one was that if left at 5 ft, it wouldn't pick up Huge creatures (15x15) unless I set it to 10 ft because it appeared to only recognize the targeting to the center of the creature and not the edge of it.
I do use the Token Height Indicator extension and it works great and how it should with your aura extension.