Discount applied to each product not cart?

Hi
In my website, if I set a 15% discount for example, it seems that the discount is applied to each item separately rather than on the cart total.

This works out slightly different because on a item by item bases, we need to round up/down each item. Which. Means the discount value can we put quite a bit depending on how many items the user is purchasing.

Is there anyway to set it so that it uses the subtotal?

Thanks