An air carrier I serviced in the early 1960's at KJFK was operating B.720 trans-Atlantic. They had JT3C non-fan engines and were marginal as far as their range for that service. They also suffered from usually carrying full pax loads being very popular with one particular ethnic group in New York. As a result we (maintenance) had to drip-stick the fuel on board to get an accurate quantity taking pitch and roll in consideration and had to check the density of the fuel on board and of the uplift fuel.
We were given the desired total fuel load, like 42,386 kilos. They had weighed the baggage and a good idea of the pax weights and the resulting fuel load was the difference between aircraft, pax and baggage weight and the MTOW. Using graphs and charts we calculated the uplift to the gallon. Literally, every extra kilo of pax or baggage meant one less kilo of fuel and a possible fuel stop. Marginal indeed.