Thanks Roger, I will investigate that.

Thanks for the reply Charles.

For the first month, the prior balance would be zero. So the available funds
would be 0 + DepositAmt.
Therefore, the balance for the first month would be IIF(Sum(ExpAmt)<=(0 +
DepositAmt),((0 + DepositAmt) - Sum(ExpAmt)),0). That balance would become
the 'PriorBalance' for the following month.

BankId is the PK for tblBank.
There can only be one record for each combination of PersonID and MonthDate.
Since MonthDate will always be the first of the month, there will only be
one record in tblBank for each PersonID for each month.
Therefore BankId provides a single field identifier for each combination of
PersonID/MonthDate.
DepositAmt is the amount added to the 'Bank' each month.

Here is the actual application. It is a method of reimbursing salespeople
for their expenses.
A new salesperson joins a company in Jan, and a 'Bank' is started with an
initial 'Deposit'.
Since they are new, there is no 'Prior Balance', so the 'Available Funds' is
equal to the 'Deposit'.
The salesperson has several expenses for Jan.
If the expenses total less than or equal to the 'Available Funds',  they are
subtracted from the 'Available Funds' and the result is the 'New Balance'
for Jan.
If the expenses are greater than the 'Available Funds', then the 'New
Balance' for Jan is zero.

Whatever the 'New Balance' for Jan is, it becomes the 'Prior Balance' for
Feb.
In Feb, the 'Available Funds' are equal to the 'Prior Balance' plus the new
'Deposit' for Feb.

So, a salesperson can be reimbursed for their expenses for each month, up to
the amount of their 'Available Funds' for that month, but they can not carry
over expenses to another month.