The seat bucket counts are the best you can do -- they tell if you there are more or less than 9 seats still to sell at each available fare bucket ... when Y is under 9, you know the flight is damn near full. AS more buckets hit zero on any given flight, you are getting closer and closer to a fully sold plane. That being said, it's not perfect as the airline sometimes blocks some seats which it will later unblock -- but it does give you a sense.