I tried the formula and noticed a few things:
1) the discount is not being displayed as Discounts on the cart created by the display manager. In this case, even though the calculation can be done in the calculation tab, it is important that is shown as discount.
2) I added one more item to the products to sell, I told you first I only had one item, now I have two sorry, and I see that the calculation is messed up, is doing the discount on the first and seconds items at the same time or something weird.
I guess I will need to open a ticket to further look into this. Sorry.