So actually, you're thinking of the mag cal, which works in any ground attitude, just as long as the attitude display is correct when you calibrate.
As for fuel quantity, it's literally taking voltage/resistance readings at each float or capacitance level that's demonstrated. If you do the cal with the tail down, you'll get slightly different readings in level attitude (depending on the aircraft's design), and vice versa. Basically, if the fuel level moves as far as the senders can see, so does the fuel quantity reading.