If you want the same ratio in the pillow's size, then you also have to take the seams into account, just like you do when re-sizing a quilt block. With quilting you would use 1/4" seams, but with a pillow I would think you would take bigger seams. Do you want the pillow's long finished side to be 15, or do you want the unfinished side to be 15? How big are your seams? There's not enough information here to give you an answer, but if you're not concerned about the seams then bearisgray has the right answer. (Although I don't know what she means by those last two lines...)