"Current Reservation %" column in the RDS Instance Asset report (https://apps.cloudhealthtech.com/assets/aws/rds/instances) is computed based on the number of hours that are 'Reserved Instance Hours' out of the 'Total Hours' the instance has ran in the current month.
Current Reservation % = (Reserved Instance Hours / Total Hours)
You can find the 'Reserved Instance Hours' and 'Total Hours' numbers that are used in the calculation in the Usage RDS Instance Hours report:
https://apps.cloudhealthtech.com/reports/usage/rds/instance-hours
The percentage should update whenever the instance metrics report is updated. Expect the percentage to update throughout a given day.