|Hold the shift key down when positioning something on the image using your mouse. This is a fairly common Windows key combination in various image editors, allowing exact positioning of objects, as opposed to the 'snap to grid' that APPX does by default.
The row and column position on the widget will still be the nearest row/col intersection, but the micro adjustment field will be filled in with the percentage adjustment away from that row/col. The same thing applies to drawing buttons, etc. - without the shift key you get 'snap to grid', but if you use the shift key, you can make it any size.