On reflection just boosting the spec value breaks the PBR model, as the upper surface of skin is a different optical properties than the lower maybe it cannot be done off just one node which means either layers or cycles node. The idea of using the physical root node is to produce a simple unbiased skin mat that is consitent across different lighting sets. The problem I found is the cross polarized image was much darker than expected (loss in the capture process ?) - typical the spec component should be around 4% but I boosted it much higher for above. Back to the drawing board but curious if any thoughts on PBR and skin mats.