Calculate account Balances for a PO system

[John]

I am creating a PO module that interfaces with an accounting system.
I have a table ("tblAccounts") that contains ministry account numbers
("AcctNumber" a text field), ministry annual budget amount ("AcctFYBudget")
and ministry remaining budget ("AcctBalGross") from the accounting system.
The “AcctBalGross†field does not take into account outstanding Pos for this
account.

I have a form for creating a PO ("frmPOs"). It contains a subform
("frmPOLines") used for inputting line items. This subform contains a combo
box ("AcctNumber") for users to input the ministry account number to be
charged for that particular line item. There are also fields on this subform
for the dollar amount to be charged ("AmtNotExceed") and a check box to
indicated if the PO line item is closed ("LineStatus"). This information is
captured on a table ("tblPOLines")
The default status of the “Line Status†check box is not checked (or “Noâ€).
The finance person checks it when they pay out on a PO and by definition, it
reflects in the “AcctBalGross†field because a draw has now been made.

I need to calculate a field on my “frmPOLines†subform that states the net
ministry account balance. This is net of outstanding POS.

(AcctBalGross) minus the sum of “AmtNotExceed†field from the “tblPOLinesâ€
table for line items associated with the account entered in the “AcctNumberâ€
combo box. However, I need to sum only those where the “LineStatus†field in
the “tblPOLines†table is “Noâ€.

I can use some help with the event.

Thanks in advance.