That’s true, and in this case where the layer is a single molecule thick, pores and even cellular structure will add to it quite a bit. Hell, at that scale it’s probably hard to define any solid boundary to the body at all, since you’ll have things like the surface of evaporating sweat. Once again, we need to know a bit more about how the magic works to give a single answer.

Our mathematician would have to add a measure on subset boundaries I guess. Or maybe just hand the problem off to a big boy who can handle things in the real world (zing!).