You can't, really. Either it's set to World alignment and stretches to line up with the X or Y axis, or it's set to Face and has a proper square aspect ratio but where it starts and ends is unpredictable. If you want it to line up with an adjacent face, click that face in the texture alignment dialog and then Alt-mouse2 on the target face. This will force the target face to use a square aspect ratio, if that matters.