如何设置XAML宽度的百分比？

I am trying to create a button in XAML with a 80% width, but I can't seem to figure out how. It's apparently not as easy as using Width="80%". I have been thinking this can be done by detecting the screen width somehow and multiply that by 0.8 and use that as the width, but I am not sure how I can do this in XAML. Perhaps this has to be done in the .cs file and then adjust the width from there. Does anyone have a solution for this?I am trying to create a button in XAML with a 8