It will be much less then 50% in this case. The main path for the grounded conductor current is the feeder neutral to the panel. The secondary path would be via the feeder equipment grounding conductor, branch circuit grounded conductor and the branch circuit equipment grounding conductor. While the resistance for this path may be small it will be much larger then the main path. It is the relative relationship between the resistance's of the paths that determine the current flow. The secondary path could easily have 10 to 20 times more resistance then the main path. A #12 has 10 times the resistance of a #4 and 20 times the resistance of a 1/0. If the path has 10 times the resistance, then only 1/10 of the total current will flow in that path.
Don
