Well, my room is about 3000 sq ft and I also sit 8 ft away. The music I am currently listening to is registering 73db on my SPL meter. Granted this is casual music listening i suppose an action movie with explosion and such would be averaging out slightly higher, but I would estimate I wouldn't be any higher than you at 85db. That being said, if 90wpc is good enough for you I would assume the same for me?

Am I understanding the equation correctly?