Active code in XAML properties

I have a situation where I want UIElement to have a 开发者_如何学Gotop margin of

((this.height / 2) - 70) 

Currently, the XAML property is

Margin="0, 40, 0, 0"

How can I combine the equation above into my XAML? I know this is probably simple, but I can't figure out what terms to search for.

Have a look at ExpressionConverter from Josh Twist -

http://www.thejoyofcode.com/The_Binding_you_wanted_from_day_one_in_WPF.aspx