so you would have the discountPercentage1, discountPercentage2, and discountPercentage3 columns
in the add to cart button behavior, on the bindings panel. you would add the percentages 0.1, 0.2, 0.3 to the corresponding columns.
then you would change the calculation to assign the appropriate discount based on quantity:
(abs([Quantity] >= 5 && [Quantity] < 11)?[Price] * [Quantity]) * [discountPercentage1]:0) + (abs([Quantity] >= 11 && [Quantity] <= 16)?[Price] * [Quantity]) * [discountPercentage2]:0) + (abs([Quantity] >= 16)?[Price] * [Quantity]) * [discountPercentage1]:0)