On 747-100/200 aircraft, the "detotalizer" was on the flight engineer's panel. The image below is, I believe, from a 747-400 because those on the -100/200 aircraft were mechanical digits rather than electronic, but I couldn't find an image of the earlier version.
Before engine start, the f.e. set the aircraft gross weight and the total amount of fuel. From that point, the instrument kept track of of your decreasing gross weight and fuel quantity.
The amount of fuel showing on the detotalizer was considered to be more accurate than that shown by the fuel quantity gauges, and thus was controlling for en route flight decisions.
Besides quickly and accurately giving the amount of fuel remaining in general, it was critical to redispatch operations (sometimes called reclear). See the "Reserve reduction" section of the Wikipedia article on flight planning for a detailed redispatch explanation.
Basically, though, redispatch allows you to legally get to a destination for which you can't (or don't want to) carry enough fuel to to reach that destination with the required reserve fuel. What happens is that you're dispatched to a destination short of your desired destination. Then, nearing the destination to which you were dispatched, you are then redispatched to the desired destination.
An example. I used to fly JFK to Tel Aviv, a flight that always required a redispatch in a -100 and sometimes in a -200. The flight was usually between 10 and 12 hours, depending on winds, how you hit the North Atlantic track system, and routing through Europe. Let's say the expected fuel burn was 275,000 lbs. The dispatch fuel requirement would be that plus 5% reserve (from memory, might have been a higher percentage). So, to be legally dispatched to Tel Aviv, you had to have 288,750 lbs of fuel at takeoff. But let's say the most fuel you could load (or wanted to) was 280,000 lbs.
No problem. You dispatched to Athens, well short of Tel Aviv, and for which the fuel requirement was considerably less than the 280,000 lbs you had on board. You used a route that took you over the Mediterranean on the way to Tel Aviv through an intersection abeam Athens. You had two flight plans aboard: one from JFK to Athens and one from that intersection to Tel Aviv. The fuel requirement for the second one was the burn from the intersection plus 5%. Thus the reserve fuel requirement for redispatch was 5% of something like 30,000 lbs (burn from the intersection to Tel Aviv) versus 5% of 275,000 lbs (burn from JFK to Tel Aviv).
Coming up on the intersection, you looked at the detotalizer. If it showed at least the required legal fuel on the second flight plan, you called the company and were redispatched. If it showed less, you would turn left at the intersection and refuel at Athens. As I remember, I never had to do that, although, truth be told, there was the occasional fudging of a hundred pounds or two, maybe three, but never more than that.
