Typically one of tthe gates A21-A26, bottom right of page 4 of this terminal map.

The Lufthansa Lounge is almost directly above this gate. The windows face south, so are shaded by louvres, but you have a good chance of seeing the plane from the lounge. If you can walk at an average pace, I would allow 5-10 minutes to reach these gates from the lounge. If you don't have any LH/SK/*A privileges it costs £30 to use the LH lounge.