A method of assigning numbers to flights includes displaying, in a first window that is displayed on a GUI, a slideable scale that receives a pairing preference input that indicates a user preference relating to the classification of flights as thru flights relative to the classification of flights as out-and-back flights. The method also includes: after displaying the first window, receiving by a processor the pairing preference input; in response to the receipt of the pairing preference input, classifying, using a flight number optimization model, each flight as one of: a thru flight, an out-and-back flight, and a stand-alone flight based on the pairing preference input; assigning, using the flight number optimization model, a number to each flight; and displaying, in a second window that is displayed on the GUI, at least a portion of an output schedule that comprises assigned numbers for each flight.