help getting FREE SHIPPING coupon to only apply to allowed items in the cart

I am working on a project where they are wanting to set up a coupon code to apply free shipping to only a particular set of items.
We have gotten the restrictions working fine to where you can only use the coupon if you have an allowed item.

Our issue is that there may be times where the customer orders other items in addition to the product on the coupon. Currently if we have free ship enabled, it applies to the entire order.

I need it to only deduct the shipping costs of the allowed coupon items, leaving any remaining items to be charged shipping as normal.

I have no problems with getting into and editing the code, I just need some direction as to what files would control this and what functions / code i would need to mess around with.

Any help would be much apprecieated. Thanks in advance!