Item Ledger Entry Table

Hi,

In Item Ledger Entry table, there is a field called source name which displays customer name. But it is only doing for document type called as sales shipment. I did opened the Item Ledger Entry Table, look inside the table, there is a field called source name, with field no as 50100, when i look at its property, it shows me like this

Field Class is: Flow Field
Calc Formula is: Lookup("Sales Shipment Header"."Sell-to Customer Name" WHERE (Sell-to Customer No.=FIELD(Source No.),No.=FIELD(Document No.)))

now, in item ledger entry table, the document type sales credit note does not show any customer name in the source name, so how can change so that no matter what document type it is, navision should be able to show me the customer name in the source name filed for entry type SALE.

Pls help!

Regards
KH