Officially, it depends upon what type of line was used. If the toe line is a raised oche, the proper distance is measured to the back side of the oche. The back side is the side furthest from the dart board where the player's foot will rest against it. Players may not stand on the oche.
If the toe line is a length of tape or other flat type of mark on the floor of a noticeable width, the proper distance is measured to the front side of the mark. The front side is the side of the toe line closest to the dart board. In such a case the player may stand on but not over the line.