Circle panel child alignement

Dear all,

I am using the follwoing sample class to align items around a radial panel as follow :

 class SolatysCirclularPanel : Panel
    {
        [Category("Common Properties")]
        public double OuterRadius { get { return (double)GetValue(OuterRadiusProperty); } set { SetValue(OuterRadiusProperty, value); }}
        public static readonly DependencyProperty OuterRadiusProperty = DependencyProperty.Register("OuterRadius", typeof(double), typeof(SolatysCirclularPanel), new UIPropertyMetadata(0.0, (o, e) => { (o as SolatysCirclularPanel).Width = (double)e.NewValue * 2; (o as SolatysCirclularPanel).Height = (double)e.NewValue * 2; }));

        [Category("Common Properties")]
        public double InnerRadius{get { return (double)GetValue(InnerRadiusProperty); }set { SetValue(InnerRadiusProperty, value); }}
        public static readonly DependencyProperty InnerRadiusProperty = DependencyProperty.Register("InnerRadius", typeof(double), typeof(SolatysCirclularPanel), new UIPropertyMetadata(0.0));

		[Category("Common Properties")]
        public double AngleForLayout{get { return (double)GetValue(AngleForLayoutProperty); }set { SetValue(AngleForLayoutProperty, value); }}
        public static readonly DependencyProperty AngleForLayoutProperty = DependencyProperty.Register("AngleForLayout", typeof(double), typeof(SolatysCirclularPanel), new UIPropertyMetadata(0.0));

        protected override System.Windows.Size MeasureOverride(System.Windows.Size availableSize)
        {
            foreach (UIElement child in Children)
            {
                child.Measure(availableSize);
            }
            return new Size(2 * OuterRadius, 2 * OuterRadius);
        }

        protected override System.Windows.Size ArrangeOverride(System.Windows.Size finalSize)
        {
            double _incrementalAngularSpace;
            Point currentPosition = new Point(OuterRadius, (OuterRadius - InnerRadius) / 2);
            int childCount = Children.Count;
            if (childCount < 4)
            {
                 _incrementalAngularSpace = (AngleForLayout / Children.Count) * (Math.PI / 180);
            }
            else
            {
                _incrementalAngularSpace = ((AngleForLayout-180) / Children.Count) * (Math.PI / 180);
            }
            double OffsetX = 0.0, OffsetY = 0.0;
            for (int i = 0; i < childCount; i++)
            {
                UIElement child = Children[i];
                double angle = (i + 1) * _incrementalAngularSpace;
                OffsetX = Math.Sin(angle) * (OuterRadius + InnerRadius) / 2;
                OffsetY = (1 - Math.Cos(angle)) * (OuterRadius + InnerRadius) / 2;
                Rect childRect = new Rect(
                 new Point(currentPosition.X - child.DesiredSize.Width / 2, currentPosition.Y - child.DesiredSize.Height / 2),
                 new Point(currentPosition.X + child.DesiredSize.Width / 2, currentPosition.Y + child.DesiredSize.Height / 2));
                child.Arrange(childRect);
                currentPosition.X = OffsetX + OuterRadius;
                currentPosition.Y = (OffsetY + (OuterRadius - InnerRadius) / 2);
				            return new Size(2 * OuterRadius, 2 * OuterRadius);
        }
    }

This works actually fine and child are properly align around the circle. But what I am trying to reach is the possibility to define for instance an alignmenet of items within a 180°angle but from the 0 to 180 ° as shown below :

Image may be NSFW.
Clik here to view.

Actually if I define an angle of 180 for my items alignement they will apears as below :

Image may be NSFW.
Clik here to view.

How can I get them aligned as first picture ?

regards

serge