I know the theoretical capcaitiy of graphite  is  about 370 mAh g-1. the formula is 1/M*26800* n, where M is the molar mass of active material, n is the number of  trasfer electrons.

So far the actual mechanism for the sodiation in hard carbon is not well established. However, researcher suggested that first, Na+ ions are inserted between parallel graphene sheets (in the sloping voltage region) . Then, Na+ ions fill the nanopores (in the plateau region) of the disordered carbon structure. The hard carbon prepared by carbonization of glucose have delivered a specific capacity of approximately 300 mA h g-1 with a low operating potential. So I think you can take the same theoratical capacity of graphite to be the theoratical capacity of hardcarbon as there exist graphene layers in hard carbon suggested by researchers.