Cargo On Board Passenger AircraftThe cargo calculation without any improvements is a random number between 0 and the maximum amount of cargo that the aircraft can carry.
When you purchase improvements for cargo, the improvement increases cargo on your aircraft by the percentage for that improvement, on top of the original random amount of kg.
For example, if the systems original amount of cargo for that particular flight was 1500kg and you have an improvement that gives you a 5% increase, then your total amount of cargo for that flight would be 1500kg x 5% = 1500kg + 75kg = 1575kg.
Cargo On Board Cargo AircraftThe calculation is different for cargo planes.
The game uses the following formula for the minimum amount of cargo on board.
Where X = Km you fly /1100
Maximum cargo load of aircraft x (X /100)
eg: Maximum flight distance is 4800km on an A300F with a maximum cargo load of 20,000kg, so if the route was at the maximum flight distance for this aircraft of 4800km.
X = 4800 / 1100 = 4.36
Therefore calculation is: 20,000kg x (4.36 /100) = 872kg.
The cargo on board for this flight would be a random number anywhere between the minimum of 872kg and a maximum of 20,000kg.
Any improvements purchased would then be calculated on top of this random number.
Hope this helps you understand how it's worked out.