Hi Airbus 757,
holding at high altitude makes very little difference to the fuel flow in the Boeing 777.
As an example, in a B773 at a weight of 220 tons:
At 1500 ft the fuel flow is 6640 kg per hour, or 111 kg per minute.
At 30 000 ft the fuel flow is 6460 kg per hour or 108 per minute.
20 000 ft seems a good average, with a fuel flow of 6180 kgs per hour or 103 kg per minute.
Rgds.