Thanks everyone for the helpful replies. I hadn't taken wind forecasts into consideration so obviously that is an important factor. I was, indeed, referencing the B777-200 as that is one of my favourite planes and I appreciate your response Smokey.
Do airlines have a set Cost Index they set for a certain type of plane or is it standard across the fleet for that company? Can you change the CI in mid-flight?
Again, I appreciate the replies.
Cheers!