Ms Access Database Design

Hi,
I'm trying to make a database to track client rental payments.
This has to be developed in access 2000.
Please see attached table structure diagram for relationships.

Problem i'm having is how i will track the rentals for each client.
I will have a main form built from the clients table showing client specific data and then a subform detailing their payments from the rental payments table. Problem is i don't know how to go about using the subform to do all the calculations and automating most of the process. ie. the only input should be from the user detailing how much rent was paid.

The fields i want on the subform would be something like:
RentFrom, RentTo, AmountDue, AmountPaid, Balance/Owing, Total(Though i think Total may only need to appear on the mainform)......remembering that sometimes clients will be behind in rent so i need to keep track of this, and show that no rent is owing when they catch up with thier rent, or show credit if needed.


I also need the database to detect which clients are due for rent
2 days prior to thier date due and two days after.

hope that gives a clear idea of what i'm trying to do.

any thoughts/suggestions to help solve this problem would be greatly appreciated as i'm totally stumped on how to approach this.

regards,

aaron