iExpense - Expense types and Project Expenditures

Hello
I'm trying to customize the way we map Expense Types to a Proejct expenditure type.
We have one Expense type and several locations. Based on the Expense type and location, I want to write custom code to do the mapping.
(As we have about 20 locations and not feasible to create many project expenditure types for each expense type and location)
So, can anyone let me know which package I should be looking to modify.
i see that AP_WEB_PROJECT_PKG.DerivePAInfoFromUserInput calls GetExpenditureTypeMapping to determine the expense type. Even though i modify this procedure,
I dont see it being called at all during expense entry.