I bet the spl increase is purely from low frequency room mode interactions that varies with room position.
But, to your direct question, it came from this very good explanation at hyperphysics:
http://hyperphysics.phy-astr.gsu.edu/hbase/hframe.html
unfortunately, it cannot link to the exact page, so follow the main page, to sound and hearing, then to the inverse square law, then click on measurement example. That page explains the room calculations except it is in centimeters
Nor does it go in even meters.
So, I interpolated from 1m to 12 ft, about an 8 dB loss.
I don't think you can realistically count the low frequency mode caused increases.