Yes, the calculation in SlideTimer_Tick assumes that it will be in the same place, how you get around that depends on what you want it to do. If it's been moved then you may want it to close straight away without sliding, in which case you can test the window position and just close it in the Hide sub.
Or you may want to prevent it being moved from it's extended position altogether, in which case you could change the window decoration so it can't be dragged, or add change listeners to the slide panel's x and y properties and set it so it doesn't move.