top of descent point- (flightlevel x 3) + 10 miles. eg FL350, (35x3)+10 = 115 miles.
B747 fuel requirements- estimated flight time to destination x 12 (-100), 11 (-200/300), 10 (-400) + your diversion requirement + 2500kg taxi/APU allowance.
These always work out to within a % or so of the computer generated plans value. Nice to know!