sorry, was missing some parentheses in the calculation code, the correct calculation code should be:
(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))