>>96525475
Generally speaking, 'normal' skin isn't going to provide any DR, regardless of how large the creature is (and thus how proportionally thick the epidermis is) unless you get to truly massive sizes. For things like toughened carapaces, scales, leathery hides, bony plates, etc, those are easy enough to base on their component material, like you mention with LT Instant Armor. But unless you're dealing with fantastical monsters with exotic physiologies, more than DR 3-4 would be extreme. Maybe something like DR 5-6 for a thick exoskeletal shell like a turtle/armadon.