Gross Profit UDF on Order

Hi,

I created 2 UDFs on the sales order document, U_FreightExpense and U_EstimatedGrossProfit. I want to enter an estimated freight expense in U_FreightExpense and want U_EstimatedGrossProfit to calculate (Gross Profit - Total Freight - U_FreightExpense) by FMS in real time. How is this possible? I'm trying to find the gross profit system variable.