I’m sorry, I guess I should have provided a more complete description. You also need to set go:LinkPanel.Index, or else it will always be at the midpoint.
I think that will position the label at 3/4 * 1/3 == 1/4 of the whole distance, because the automatic calculations of the control points of a Bezier link are at 1/3 and 2/3 of the total distance.