Unfortunately the devs never fully disclosed the info. This is the best they have explained Maximum positions in guild war rating - #2 by Kafka
But i think your score is also compared to other brackets, as bracket 50 is theoretically easier than bracket 45, and 1M points in bracket 50 is considered less than 1M points bracket 45.
Either way, just try to score as high as you can and youll move up faster. 1 guild reported they moved up 19 brackets Guild wars bracket placements - #2 by Ashasekayi