Dear antimatter1,
The acceptance rate is the number of applicants admitted divided by the number of people that apply. There is no one component of the application that is weighted more heavily than another, so you cannot really apply the acceptance rate to the GMAT score in this manner. If a school's average GMAT is a 700, then that means it is the average for the people who actually enrolled in the program--not that were admitted. The number that enrolls is always less than the number admitted. Likewise, the mid-50% is the range for the entering class, not the mid-50% range of the admitted applicants.
Regards,
Lisa