Payment Calc In Table

This is the second edited section:
I got the update query to work properly. Is this the best way to solve my problem?

This is the original edited section:
For the problem below would it be possible to use an update query to update the table?
Criteria for the termination_date field would be "is not null"
payment_to_date Update to: field would equal DateDiff("m",[payment_start_date],Now())*[monthly_payment]
Is it possible to use other fields in the table as part of the UpdateTo: calculation? I get an error when I try this.

This is the original question:
I have a field in a table called payment_to_date that I want to always be populated with the current total of how much a customer has paid to date based on their monthly payment rate. I have a form for data entry into this table but once their account info is entered the first time their won't be any changes to the account unless they terminate the service so calculating from an event procedure on the form doesn't make sense to me (I am new at this).

I entered the following in a text box control source on the form and it works but the textbox is not bound to the table so it's not the right way to do it.

=IIf([termination_date] Is Null,(DateDiff("m",[payment_start_date],Now())*[txtpayment]),(DateDiff("m",[payment_start_date],[termination_date])*[txtpayment]))

The calculation from above is basically:
If there is no termination date then the payment to date = the number of months between todays date and the date payment was started multiplied by the monthly pay rate.

Can anyone tell me how to keep the information in the table current for the payment_to_date field?

Any help is appreciated.

Troy