Custom Field Price adjustment ignores discount

[Nocktis]

I'm trying to create a few custom fields for shirt size (small, medium, large, x-large, xx-large).  If I have the product set to on special and apply a +1.00 to the XX-Large shirt, the +1.00 is added to the base price and not the sale price.  Is there another way to do this?

For example:
Sales price: $10.00
Discount: $8.00

Select the XX-Large shirt with a +1.00 price:
Sales price: $10.00
Discount: $9.00