If you want to place a window at a specific place in WPF, it will work pretty much as you expect—unless your DPI is 120 (the default is 96). Here’s a sample that shows how to put it where you want.
In this case, I want to put a window just under another control, aligned to its left side.
The crux of this is getting the current DPI. You could use P/Invoke to call native methods to get this, but the transformation matrix contains the same information as well.