Wpf custom control template - relative font size

public class MathConverter : IValueConverter
{
    public object Convert( object value, Type targetType, object parameter, CultureInfo culture )
    {
        return (double)value + double.Parse( parameter.ToString() );
    }

    public object ConvertBack( object value, Type targetType, object parameter, CultureInfo culture )
    {
        return null;
    }
}

<my:MathConverter x:Key="MathConverter" />

<TextBlock FontSize="{Binding
                     RelativeSource={RelativeSource AncestorType={x:Type Window}},
                     Path=FontSize,
                     Converter={StaticResource MathConverter},
                     ConverterParameter=2}" />

public class FontSizeConverter : IValueConverter
{

	#region IValueConverter Members

	public object Convert(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
	{
		return (double)value + 12.0;
	}

	public object ConvertBack(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
	{
		return (double)value - 12.0;
	}

	#endregion
}

<Style.Resources>
        <local:FontSizeConverter x:Key="fontSizeConverter"/>
</Style.Resources>

<TextBlock FontSize="{Binding RelativeSource={RelativeSource TemplatedParent}, Path=FontSize, Converter={StaticResource fontSizeConverter}}"
                                   Grid.Row="0" Text="{Binding RelativeSource={RelativeSource TemplatedParent},
                                   Path=Date.Day}" HorizontalAlignment="Right" VerticalAlignment="Top" Padding="2" Margin="2" >
 </TextBlock>