Initially, I had thought that the axial load carrying capacity is the difference between the proof load and the preload (preload due to torquing the fastener), however, after more research, I believe that is not the case. I think bolt stiffness and member stiffness matter in this case, however, my engineering design book from college does not cover this scenario. It only covers cases of two members being clamped by a nut and bolt, or a member being clamped to another member by means of a bolt and threaded hole.