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.

CalcWindow calc = new CalcWindow();

Point point = this.PointToScreen(

    new Point(0, this.ActualHeight));

PresentationSource source = 

    PresentationSource.FromVisual(control);

double dpiX = 

    96.0 * source.CompositionTarget.TransformToDevice.M11;

double dpiY = 

    96.0 * source.CompositionTarget.TransformToDevice.M22;

calc.Left = point.X * 96.0 / dpiX;

calc.Top = point.Y * 96.0 / dpiY;

calc.Show();

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.