Volume discount question

Based on testing, it seems to me the system decides on the discount in this order:

1. Special prices for BP

2. Discount group

3. Period or volume discount

4. Price List

This is my scenario:

I have customers with a discount on a certain product and I am putting it on special with a quantity buy but the lower discount populates rather than higher quantity.

1. Special prices for BP - NONE

2. Discount group - 15% on item group

3. Period or volume discount - on base price list: 20% when Qty is 5

4. Price List

Enter sales order - Qty 5 - populates 15% discount.

I want to give the highest discount i.e..20% when Qty is 5

Is there a way to make an item with a quantity buy, to supersede a discount group discount?

How can this be achieved? What is the recommended way to do this?

Thanks