raraa,
I take you have read the "Getting to grips with Airbus performance" books. They are large and boring, but mostly quite good. Googling around for the cost index one and fuel economy one:
http://www.smartcockpit.com/data/pdf...Cost_Index.pdf
SmartCockpit - Airline training guides, Aviation, Operations, Safety
In summary,
Calculate your CI. As the cost index drops below 20 or so, the fuel savings get much smaller but the time costs get much larger. The display on the MCDU might not be able to resolve the fuel saving.
Give the machine as much wind data as you can, ideally with a good wind profile at plenty of cruise points.
Fly managed speed and as close to the optimum level as possible.