Adding Total on form to calculated field

I need to add a total to a form under a grid where the total of four fields will be shown.  Three of the fields are database fields so I've added code to the executeQuery method on the form datasource to get these values (using aggregate function) and populate my new total controls with them. 

But the fourth field is based on a display method on the table.  What is the best way to get the total of this field?