Default summary calculation algorithms process values that belong to a field specified by a summary item. When calculating a summary value manually, this column's values are passed to corresponding event handlers via the FieldValue property. Use this property to access the processed value immediately and update the CustomSummaryEventArgs.TotalValue property value accordingly.
Note that sometimes you may need to include several fields in your summary calculations. In this case, you will need to use the CustomSummaryEventArgs.RowHandle parameter to identify the row being processed and access its values within the desired fields.