WPF TextBox Height Equals Parent Height minus 50 pixels?

Question

I'm successfully getting a TextBox to resize with a parent Border height but I need the TextBox to actually be 50 pixels smaller in height than the parent Border.

Any ideas how to achieve this?

The code I'm using is

<Border VerticalAlignment="Stretch" HorizontalAlignment="Stretch" >
     <TextBox x:Name="txtActivityNotes" HorizontalAlignment="Stretch" Height="{Binding RelativeSource={RelativeSource FindAncestor, AncestorType={x:Type Border}}, Path=ActualHeight}" AcceptsReturn="True" VerticalContentAlignment="Top" TextWrapping="WrapWithOverflow" VerticalScrollBarVisibility="Auto" />
</Border>

Answer 1

how about using a converter on the binding to minus 50 off the height

heres an example of using a converter

Answer 2

Can't you just set a bottom margin of 50?

<TextBox Margin="0,0,0,50" />