Code

void ArrangeBackLinks(FlowchartLayout layout)
{
	Dictionary<int, List<DiagramNode>> levels = GetNodesByLevel(layout);
	foreach (List<DiagramNode> level in levels.Values)
	{
		RectangleF levelBounds = LevelBounds(level);
		foreach (DiagramNode node in level)
		foreach (DiagramLink link in node.OutgoingLinks)
		{
			if (link.Origin.Bounds.Y > link.Destination.Bounds.Y)
			{
				float x = GetTopCenter(link.Origin).X - levelBounds.Left;
				SetLoopPath(link, levelBounds.Right, x / levelBounds.Width, layout);
			}
		}
	}
}

void SetLoopPath(DiagramLink link, float x, float offset, FlowchartLayout layout)
{
	PointF origin = GetBottomCenter(link.Origin);
	PointF dest = GetRightCenter(link.Destination);

	float availableSpace = Math.Min(layout.NodeDistance, link.Destination.Bounds.Height);
	offset *= availableSpace;

	float top = GetTopCenter(link.Destination).Y + offset;
	float right = x + availableSpace - offset + layout.LinkPadding;
	float bottom = origin.Y + availableSpace - offset;

	PointCollection points = link.ControlPoints;
	points.Clear();
	points.Add(origin);
	points.Add(new PointF(origin.X, bottom));
	points.Add(new PointF(right, bottom));
	points.Add(new PointF(right, top));
	points.Add(new PointF(dest.X, top));
	link.UpdateFromPoints(false, true);
}

Dictionary<int, List<DiagramNode>> GetNodesByLevel(FlowchartLayout layout)
{
	Dictionary<int, List<DiagramNode>> levels = new Dictionary<int, List<DiagramNode>>();
	foreach (DiagramNode node in diagram.Nodes)
	{
		int level = (int)GetRightCenter(node).Y / (int)layout.NodeDistance;
		if (!levels.ContainsKey(level))
			levels[level] = new List<DiagramNode>();
		levels[level].Add(node);
	}
	return levels;
}

RectangleF LevelBounds(List<DiagramNode> level)
{
	RectangleF bounds = level[0].Bounds;
	foreach (DiagramNode node in level)
		bounds = RectangleF.Union(bounds, node.Bounds);
	return bounds;
}

private PointF GetLeftCenter(DiagramNode node)
{
	RectangleF r = node.Bounds;
	return new PointF(r.Left, r.Top + r.Height / 2);
}

private PointF GetRightCenter(DiagramNode node)
{
	RectangleF r = node.Bounds;
	return new PointF(r.Right, r.Top + r.Height / 2);
}

private PointF GetBottomCenter(DiagramNode node)
{
	RectangleF r = node.Bounds;
	return new PointF(r.Left + r.Width / 2, r.Bottom);
}

private PointF GetTopCenter(DiagramNode node)
{
	RectangleF r = node.Bounds;
	return new PointF(r.Left + r.Width / 2, r.Top);
}