Nathaniel's highest advanced skill would be A Scholar of the Correspondence if it was an advanced skill, and it would be something in the range of like. 25. (his Artisan of the Red Science is notably lower - he works more with the concepts and theories of the Correspondence rather than its potential uses in the breaking of reality. a mathematician rather than an engineer, in contrast to the doctor)
as it stands, his actual highest advanced skill is Mithridacy, thanks in large part to a lifetime lived under the thumbs of people with moods more volatile than the H.M.S. Inadvisable