How can I configure AR to automatically write-off small over/under payments?

Good afternoon.  

Our system was configured for Payment Predictor to only match exact items.   We now would like Payment Predictor to automatically write-off small over/under payments.  I have done several different set-up changes, and nothing seems to be working.   I am fairly new to the AR module, but have been using PeopleBooks, this site, and other set up documents to get this to work correctly.   I would appreciate any information you could provide. 

Thanks!

Diane