Handy tip, if your bouncing a stereo pair or mastering a mix on two console channel pre amps with slightly inaccurate gain pots or hands showing the scale. Play your recording & get your balance as close as you can ignoring the numbers, then use the mono button. almost every lead vocal is placed dead centre, so as you flip between mono and stereo if your vocal doesn't shift from one side or the other and remains dead middle then you should of lined your fader and pre amp combinations correctly. Now make sure your 'artist' doesn't lightly nudge it ;)