Apply discount rates to individual cart items

Hi.
I have a simple product with a text field (product add-on) to customize each individual item added to the cart.

I have a discount table with following rates:
– from 10 units: 5% discount
– from 20 units: 10% discount
– from 30 units: 15% discount

I need it to apply the discount rate for each individual item added to the cart. Meaning:
CART:
item #1: 5 units: 0% discount
item #2: 11 units: 5% discount
item #3: 2 units: 0% discount
item #4: 25 units: 10% discount

Currently it’s applying the same discount rate (to all items) regardless of the individual quantities, and the rate applied is the one determined by the quantity of the last item. In the example it’s applying 10% discount to each individual item.