8:1 is just the common middle ground that has been accepted, but oratory goes into it a little more in section 3 of this post https://www.reddit.com/r/oratory1990/comments/ugofb0/why_can_measurements_from_different_sources/

Tl;dr while 8 times is the generaly accepted ratio, there's no reason its specifically that, as even an 8:1 ratio can cause a 1dB difference in the frequency response of the headphones due to the damping factor. so a higher ratio can still be more desired for less of a change in FR