How do I bind to a property?
Here's my simplified scenario.  I can adjust my inner rectangle width using the mouse.  A textblock displays the width which changes as I adjust it.  I want a second textblock to display the value of a property which also changes with the width but I cannot figure out how to bind it.

    <Grid x:Name="LayoutRoot" Background="White" VerticalAlignment="Center">

        <Rectangle x:Name="aRec" Height="100" Width="100" MinWidth="10" Fill="Blue" />

        <Rectangle x:Name="myRec" Height="100" Width="300" MinWidth="10" Fill="Red" Opacity="0.5" 
                   MouseLeftButtonDown="myRec_MouseLeftButtonDown" 
                   MouseLeftButtonUp="myRec_MouseLeftButtonUp" 
                   MouseMove="myRec_MouseMove"></Rectangle>

        <StackPanel>
            <TextBlock x:Name="myText1" Width="40" Height="20" Foreground="White" Text="{Binding ElementName=aRec, Path=Width}" />
            <TextBlock x:Name="myText2" Width="40" Height="20" Foreground="White" Text="{Binding Value}" />
        </StackPanel>
    
    </Grid>

    public partial class MainPage : UserControl
    {
        Boolean active = false;

        private Double _value;
        public Double Value
        {
            get { return _value; }
            set { _value = value; }
        }

        public MainPage()
        {
            InitializeComponent();
        }

        private void myRec_MouseLeftButtonDown(object sender, MouseButtonEventArgs e)
        {
            active = true;
        }

        private void myRec_MouseMove(object sender, MouseEventArgs e)
        {
            if (active == true)
            {
                aRec.Width = e.GetPosition(myRec).X;
                _value = aRec.Width * 10;
            }
        }

        private void myRec_MouseLeftButtonUp(object sender, MouseButtonEventArgs e)
        {
            active = false;
        }          
    }
How do I bind to a property?
===

Here's my simplified scenario.  I can adjust my inner rectangle width using the mouse.  A textblock displays the width which changes as I adjust it.  I want a second textblock to display the value of a property which also changes with the width but I cannot figure out how to bind it.

    <Grid x:Name="LayoutRoot" Background="White" VerticalAlignment="Center">

        <Rectangle x:Name="aRec" Height="100" Width="100" MinWidth="10" Fill="Blue" />

        <Rectangle x:Name="myRec" Height="100" Width="300" MinWidth="10" Fill="Red" Opacity="0.5" 
                   MouseLeftButtonDown="myRec_MouseLeftButtonDown" 
                   MouseLeftButtonUp="myRec_MouseLeftButtonUp" 
                   MouseMove="myRec_MouseMove"></Rectangle>

        <StackPanel>
            <TextBlock x:Name="myText1" Width="40" Height="20" Foreground="White" Text="{Binding ElementName=aRec, Path=Width}" />
            <TextBlock x:Name="myText2" Width="40" Height="20" Foreground="White" Text="{Binding Value}" />
        </StackPanel>
    
    </Grid>

    public partial class MainPage : UserControl
    {
        Boolean active = false;

        private Double _value;
        public Double Value
        {
            get { return _value; }
            set { _value = value; }
        }

        public MainPage()
        {
            InitializeComponent();
        }

        private void myRec_MouseLeftButtonDown(object sender, MouseButtonEventArgs e)
        {
            active = true;
        }

        private void myRec_MouseMove(object sender, MouseEventArgs e)
        {
            if (active == true)
            {
                aRec.Width = e.GetPosition(myRec).X;
                _value = aRec.Width * 10;
            }
        }

        private void myRec_MouseLeftButtonUp(object sender, MouseButtonEventArgs e)
        {
            active = false;
        }          
    }