How to use balance to new month in AR(account receive)

[peterth]

Hi All,

May I have concept or direction of A5V10 command for simple Account Receive. If I have AR table , within AR form to calculate these.
1. calculate all debt of last month
-All Debt: use DebtLast = tablesum( invoice_items.dbf)
(calculate field rule )
2. calculate receive money from member
-Paid Amount: use rec_paid = tablesum(AR.dbf)
(calculate field rule , a field in AR table)
3. calculate the balance
-Outstanding Balance: use ar_balance = rec_paid - DebtLast

The problem is how can I use Oustanding Balance (this month)
as previous balance for next month


AR form
--------
Previous Balance $300
Paid Amount $300
Post-Bill Adjustment $0
----------------------------
Current Debt $500
Outstanding Balance $500 --> (if member paid 400, ar_balance will be $100 and will be the previous next month)



AR table
-----------------
Date Previous_bal debtlast Rec_paid ar_balance
15/3/2010 1000 800 -200 -->(800-1000)
15/4/2010 200 1500 1700 0 -->(1700-200+1500)
15/5/2010 0 500 450 -50 -->(450-500)
15/6/2010 -50

The problem is how can I use the AR_balance the current month as the Previous_Balance for next month ?
There is any command to get the value from last record/last month to put
as the default value in the Previous_balance field.

Thanks
Peter-th