on the column tab, you need to create a new column to flag items the discount should apply to. set the column name to something like "discountable" for items the discount applies to, set the value to 1, for items the discount does not apply to set the value for the discountable column to 0.
in the discount calculations, set the calculation to "Based on items with a specific value":
Total of 'TotalPrice' where 'discountable' has a value of '1' 'times' '.15'