In the EC2 Instance Hours report, the "% Used" number is calculated by dividing the number of hours a specific instance ran during a certain month by the total hours in that month.

For example, If an instance ran for the entire month of September, it would have been running for 720 hours, so the "% Used" would be 100%. If an instance was running for 550 hours, that would be 550/720 so "% Used" would be 76%.