The best way I like to scratch the glass on my calls are at the 7-10 o'clock position of the call. Scratch going up & down about 1-2 inches, then scratch going left & right so you are creating a checkerboard like scratch with the sandpaper. Keep repeating this until you get the sound you like.

Note: keep sandpaper with you while out in the field. If your call gets wet or the glass calling surface smooths out, you will want to rough up the glass again.