as i know
if u configure over due
entry 1- from 1 to 15 overdue as 1 , fine will be calculated 1rs per day after the due date till the end of 15days
entry 2 - from 16 to 30 overdue as 5, fine will be calculated 5rs per day after 15 days of the due date
for example if the book overdue for 20 days fine will 90- 15as per 1st entry +75rs as per 2nd entry