Currently I do fuel planning as follows:

From several flights I obtained the following empirical numbers for fuel flow during cruise flight:







td] [/td]

Fuel Flow (1,000 pph)	Speed (M)
Altitude (FL)	0.63	0.66	0.70	0.80	0.82	0.827	0.83
100	21.2
150		19.2
250			14.4	18.8
270				17.2
330			11.6	12.8	14.4		15.6
350					13.6
370			9.6	11.8	12.0	12.6
390				10.0	11.2
410				9.6	10,4

01) [Speed of Sound at cruising altitude (kts)]
= IF [Flight Level] > 370 THEN 573.17 ELSE (-0.2378 * [Flight Level] + 660.98) END

02) [True Cruising Airspeed (kts)]
= [Mach Cruising Speed]
* [Speed of Sound at cruising altitude]

03) Estimated [Climb, Descent & Approach Distance on course (nm)] = 130

04) [Cruising Distance (nm)]
= [Flight Plan Distance (nm)]
- [Climb, Descent & Approach Distance on course (nm)]

05) [Time of Cruise Flight (hrs)]
= [Cruising Distance (nm)]
/ [True Cruising Airspeed (kts)]

06) [Cruise Fuel (1,000 lbs)]
= [Cruising Fuel Flow (1,000 pph) from table]
* [Time of Cruise Flight (hrs)]

07) Estimated [Taxi-out Fuel (1,000 lbs)] = 1.3

08) Estimated [Takeoff & Climb Fuel (1,000 lbs)] = 12

09) Estimated [Descent & Approach & Landing Fuel (1,000 lbs)] = 5

10) Estimated [Taxi-in Fuel (1,000 lbs)] = 0.9

11) [Fuel Required (1,000 lbs)]
= [Taxi-out Fuel]
+ [Takeoff & Climb Fuel]
+ [Cruise Fuel]
+ [Descent & Approach & Landing Fuel]
+ [Taxi-in Fuel]

12) [Diversion-to-Alternate Fuel (1,000 lbs)]
=
(
IF ([Distance to Alternate (nm)] - [Climb, Descent & Approach Distance on course (nm)] < 0
THEN 0
ELSE
([Distance to Alternate (nm)]  - [Climb, Descent & Approach Distance on course (nm)]
END
)
/ [True Cruising Airspeed (kts)]  
* [Cruising Fuel Consumption (1,000 pph)]
+ [Takeoff & Climb Fuel (1,000 lbs)]
+ [Descent & Approach & Landing Fuel (1,000 lbs)]

13) Estimated [Holding Fuel (30 min @ 225 KIAS, 1,000 lbs)] = 5.2

14) [Safety Addition (10%) (1,000 lbs)]
= 0.1
*
(
[Fuel Required (1,000 lbs)]
 + [Diversion Fuel (1,000 lbs)]
 + [Holding Fuel (30 min @ 225 KIAS, 1,000 lbs)]
)

15) [Total Fuel (1,000 lbs)]
= [Fuel Required (1,000 lbs)]
+ [Diversion Fuel (1,000 lbs)]
+ [Holding Fuel (30 min @ 225 KIAS, 1,000 lbs)]
+ [Safety Addition (10%) (1,000 lbs)]



I do not take into account
- the actual weight of the airplane
- cruise climbs
- actual pressure altitude
- ...
- ...
but it works well enough.

--------------
Markoz:
What altitude at what weight?

Derived from the Pan Am 707-300 Aircraft Operating Manual:

0.82M Cruise Optimum Altitudes:
Gross Weight (GW) <= 205,000 lbs : FL410 / FL390
GW <= 265,000: FL370
GW <= 275,000: FL350
GW <= 315,000: FL330 / FL320

Long Range Cruise Optimum Altitudes (0.805M … 0.806M):
GW <= 195,000: FL410
GW <= 215,000: FL390
GW <= 275,000: FL370
GW <= 295,000: FL350
GW <= 315,000: FL330




Best regards
Alex