I've just completed a relatively short sector (1518 nm) in a B777-200 with a Lido Flight Plan showing a TIME of 3:28 and FUEL BURN of 21767 Kg.
Lido add a very nice little estra to their CFPs, quoting directly -
ONE FL BELOW BURN ADD 447KG / TIME 03:27
That's as near as dammit to a 2% increase for 1000 ft below planned (Optimum) level. Over a reasonable range of lower levels, it would be safe to extrapolate this figure, e.g. for 6000 ft below Planned Level, add 12%. This seems to be in line with previous responses.
I'm about to launch on a Trans-Pacific crossing with a B777-300ER, and with this this thread in mind will post if, for a much longer flight, the percentage increment changes significantly. (I suspect that the percentage increment will be much the same).
Maurice Chavez, Assuming that RSA indicates Saudia Arabia (and not South Africa), I suspect that your CIs from your oil-rich country would be above LRC, so not a bad suggestion. For we mere mortals from oil-deprived countries, our CIs are already below LRC, a wiser suggestion might be to reduce to Maximum Range Speed (CI=0)
Regards,
Old Smokey