Lift coefficient
The aircraft designers do have a rough idea of what the lift coefficients ought to be like. But a rough one...
Having a look at the very A380 programme:
The aircraft builders had estimated Cl value over 5 % too low. The Vref turned out to be 4 knots less than the predicted 137 knots (Does it follow that A380 will fly to runways previously thought too short?)
They got the wing strength wrong by over 2 %, too - the safety margin turned out to be 146 %, not 150 %. Does it mean A380 has lost over 2% of MTOW?
However, the Cl estimates were still within less than 10 % of actual... What are the typical Clmax values for airliners?
The Airbus engineers DID have an idea of Cl before they built the wing and chose an airport to attempt takeoff...