I am not sure where or why this thread is trying to illustrate.
Currently, I use the model expressed in the FMS..."right" or "wrong", that is what the aircraft is using. The FAA can come up with calcs to 12 sig digits, but if the FMS disco's...well....
While the FAA's calcs work in a straight line, the calcs do not work in a curve...(ie TARGETS busts)