Forget the Lift formula, drag and so on...
And the pax / cargo formulae do not apply to military jetfighters, and are thus discredited.
No - the real answer (at least for aircraft with one extra seat) lies in the ARMREST THEORY - which goes like this:
The pilot(s) taxi the aircraft onto the runway, and then push the "LOUD NOISE" lever(s) forward to make the motors much more noisy. Passengers, upon hearing this fearsome noise, grip their armrests more tightly and pull upwards. This upward force on the armrests is transferred through the structure to every molecule of the aeroplane, and thus the vehicle begins to fly.
(source: The book of Lift, chapter 29.92)