Pass the value of one control to a Converter to set the width on another control

I want to set the width of a TextBlock based on the width of its container, minus the margins set on the TextBlock.

Here is my code:

<TextBlock x:Name="txtStatusMessages" 
           Width="{Binding ElementName=LayoutRoot,Path=ActualWidth }"
                   TextWrapping="WrapWithOverflow" 
           Foreground="White" 
           Margin="5,5,5,5">This is a message
</TextBlock>

And that works great except for the fact that the TextBlock is 10 units too big due to the Left and Right Margins bbeing set to 5.

OK, so I thought... Let's use a Converter. But I don't know how to pass the ActualWidth of my container control (SEE ABOVE: LayoutRoot).

I know how to use converters, and even converters with parameters, just not a parameter like... Binding ElementName=LayoutRoot,Path=ActualWidth

For example, I can't make this work:

Width="{Binding Converter={StaticResource PositionConverter},  
       ConverterParameter={Binding ElementName=LayoutRoot,Path=ActualWidth }}"

I hope I made this clear enough and hope that you can help because Google is no help for me tonight.