You can use the LSLookup function to retrieve values in a reverse way - i.e. you supply the value for the 2nd column and get the value in the 1st column back from the function. To achieve this, you add "#REVERSE" to the Link Storage Code. For example LSLookup('CUST'+'#REVERSE',"AccountId")
##LSLookup('CUST', "No.")
This will retrieve the Account Id corresponding to the Customer No. in NAV (provided that the Link Storage CUST contains NAV Customer No's and corresponding SFDC Account Ids.
##LSLookup('ITEM',"ITEMNMBR")
This will retrieve the Product Id corresponding to the Item Number in GP (provided that the Link Storage ITEM contains GP Item No's and corresponding SFDC Product Ids.
##LSLookup('CUST'+'#REVERSE',"AccountId")
This will retrieve the NAV Customer No. corresponding to the SFDC Account Id (provided that the Link Storage Cust contains NAV Customer No's and corresponding SFDC Account Ids.
##LSLookup('SALESORDER', "Document Type"+'#'+"Document No.")
This will retrieve the SalesHeader Id corresponding to the SalesOrder Document Type and Document No. in NAV (provided that the Link Storage SALESORDER contains NAV SalesOrder Document Type + Document No's and the corresponding SFDC SalesHeader Ids.
This formula would typecally be used in the Field List on a Sales Line Transfer.