to truly calculate FC, u should do it when refueling ur fuel..
take KM traveled, divide with Liter of fuel pump..
its more accurate that way since computer doesnt calculate the fuel used during startup.. it gets more inaccurate when u reset the FC meter while driving half way..

as mirage claimed 21km/l most owner got 14-16km/l (using refuel calculation)
if they reset halfway.. the figure can go as high as 26km/l..

so is always better to do manual calculation when pumping fuel.

from a mirage owner..