on the calculations tab, create a new calculation that uses the original calculation for total price:
[Price] * [Quantity]
Then in the discounts, instead using the TotalPrice column, use the OriginalTotal column.
your calculations are set to use Items with a specific column value:
Discountable2 = 1
any items where the Discountable2 column is not equal to 1 will be ignored from the discount