Ah, optimum FLs in the A320 series. Mounts up on hobby horse.........
The FMGC Optimum FL in the 320 series depends on, as others have said, Wt (mass), CI, wind, Temp and 15 mins in the cruise. But what do we tell it? Wt - yes, CI - yes, wind - maybe, Temp - maybe one. But the CFP knows the whole dynamics of the atmosphere that day ie all the winds and temps at all the levels. Therefore, provided the CFP projected wt (mass) is close to the aircraft's actual mass, the CFP optimum will be the level that will be best for fuel in the light of the CI used.
Time and time again we see guys load up the FMGC with a crz level and temp, maybe a trip wind and then wonder why the FMGC optimum differs from the CFP optimum and elect to fly the FMGC optimum which is invariably lower thereby wasting fuel.
There is, of course, more to it than that.....dismounts.
Cheers,
mcdhu