It is not possible to directly compute the sensitivity of some result with respect to a layer thickness. This is an operation that you would, for example, need for optimization studies. There is, however, an indirect way to obtain such a sensitivity, and that is to use the Scale property available in a Layered Material Link. A scaling acts on all layers in a Layered Material Link node. In order to scale individual layers, you can use a Layered Material Stack, and add separate Layered Material Link subnodes pointing to these layers.