Something I think Kliphe should consider...unlike most folks with very constricted shooting spaces, your spacious area permits you to ignore issues present in small spaces!
- The small space shooter needs to limit the amount of unwanted bounce back of light by adjacent walls and ceiling...ergo thoughts of 18% gray walls or darker
- The large space shooter can take advantage of Inverse Square law, and ignore unwanted bounce back
- if light source is 4' from subject and 8' from adjacent wall, the light striking the wall is already -2EV in intensity before it is further reduced by reflecting off the imperfect reflective surface of the wall (losing another -1EV even from a white wall, or more), and any light bouncing back to the subject is further attenuated in the distance from wall to subject! Light at -3EV is virtually non-existent from a photographic point of view.
IOW, you can choose a brighter tone for the wall without issue!
The need for black wall is useful if doing certain types of shoots entailing open-shutter (Bulb) and/or absolute black background, but backdrops can be set for that, rather than having a permanently black wall.