Discounts For Carton Qty - Mixing Step Qty With Volume Discount

I need to set discounts for full carton/box qty of an item but can't find how to do this, can you help?

For example, Product A is stored in cartons of 12. The customer pays full price for buying 1 single (we have to re-box it) and gets a discount of 10% if they purchase in full carton qty (Step quantities of 12).

If we only sold in step quantities of 12 it would be easy but my boss wants to offer it in singles also.

Using the quantity discount feature means that they get the discount on every unit - even if they order 13 (1 full box and 1 single which costs us more to pack)

How do I configure it so that the customer can choose either the singles/split box price or the discount on step qtys?

I'd suggest you create a variation that was for 'carton' where the quantity is 12 and the step is 12.