I believe somewhere there in Section VIII, it gives an allowance for effective diameter of threaded fittings, but wasn't able to find it earlier.
Couplings do have a specified ID in the non-threaded part which is not normally shown in dimensional tables, and that could be used if the threads were beyond the area in question.
What I have commonly seen also is to assume the ID of the coupling is the OD of the pipe that would thread into it, which should be a little conservative.