Automatic Tiered Discount

I need to set up a "Buy More Save More" discount, where customers can save up to 15% off their order depending upon how many items are in their cart. 

I followed the setup tutorial on Youtube for a standard tiered discount. I was hoping that the tiered discount would apply automatically rather than the customer having to enter the code during checkout. I don't want me customers to have to enter "5-off" to save 5% on 2 items or "10-off" to save 10% on 3 items, or "15-off" to save on 4+ items.

Am I missing how to implement this correctly?

Thank you,