控件的大小绑定到上层容器的大小

<Grid Loaded="Grid_Loaded_1">
        <Grid.ColumnDefinitions>
            <ColumnDefinition Width="125"/>
            <ColumnDefinition Width="*"/>
            <ColumnDefinition Width="50"/>
        </Grid.ColumnDefinitions>
        <Grid.RowDefinitions>
            <RowDefinition Height="50"/>
            <RowDefinition Height="*"/>
        </Grid.RowDefinitions>
        <StackPanel Grid.Row="1" Grid.Column="1" Grid.ColumnSpan="2"
                    Orientation="Horizontal" >
              <Grid Width="{Binding RelativeSource={RelativeSourceMode=FindAncestor,AncestorLevel=1,AncestorType={x:Type Grid}}, Path=Width }"   
                  Height="248"   
                  Background="Blue" >
            </Grid>
            <grid>...</grid>
        </StackPanel>
<Grid>

里面的Grid的width和height如何绑定到最外面的grid的对应的布局快的大小？
也就是stackpanel相对于父节点100%，里面的grid相对与stackpanel100%
[解决办法]
ElementName:指定源对象的名称

Path ： ActualWidth