How to calculate the payment due past due date

Hi

This is my first post and I really need some help. I need to find a way to calculate the monthly charges due once it has passed the due date. These amounts are monthly charges due for service charge per resident in a housing estate. I have put in a column with conditional formatting that shows how many days the unit is past due. Since clients have the option to pay for more than a month, I have also put in columns that monitor the numbers covered under their last payment. Since the monthly charges are 17000 and we can not compute the charges based on the days past due, how do I show the exact amount due to the J column.

I have attached a copy of the spreadsheet.

Any suggestions on what formulas I can use?