Yeah, you screwed up the equation somehow. You might have messed up the Sabines/Absorbtion Co-efficient thing likely. The "Sa" part of the equation is better described in F.Alton Everest Master Handbook of Acousics. It works perfect for me every time basically.
Robert Harleys RT60 equation does not work
After reading "The complete guide to Home Theater" by Robert Harley I have noticed quite a few discrepencies. After carefully plotting in all square footages and using his equation on page 113 I got RT60 times of 0.001476 to 0.001292. I have a 6460 cubic foot room (34x19x10). All surfaces were drywall except for the floor which was carpet. How is this possible in a room of this size??