The 1/3 "rule" only comes from the fact that the sail acts like a wing and these have their pressure center positioned around 1/3 from the leading edge. It is only meant as a quick way to find the pressure point, and harness position, on the beach when changing the boom length for one sail to another. It is not meant to be a "rule" other than a "rule of thumb" and in fact it doesn't work well on race sails above 7.5m^2.
Once on the water, you adjust the harness position to suit your sailing style. Personally I mark the position directly on the boom and barely manage to use the same positions on both sides of the boom...