Use a text editor to edit the "mission_[mission number]_goal_player_0.qst" file in the 
"\Program Files (x86)\Kalypso Media\Airline Tycoon 2\init\quest\mission\<mission>" 
directory. 

Near the bottom you will see something similar to 
< value.f >0.75</ value.f >< tt >. Change the < tt >0.75 to whatever desired. 
For example, 0.10 would be a 10% goal in game. Load a saved game or start a new game.
You can also edit the AI players' numbers to change their goals in the same way.