johns7022;
The A330 FCOM gives information for holding at 'green dot' speed in clean configuration, which is a speed between minimum fuel speed and minimum drag speed.
Fuel consumption varies only slightly with altitude, for example at a typical weight it is 2038 kg/h/eng at FL15, 1980 at FL150 and 1960 at FL250.
regards,
HN39
So you fly an Airbus at FL250? If the Island up ahead is weathered in, chances are I will loiter at FL410 rather then go down and hold at FL150, but I would have to look at the actual numbers to determine that....