If you want to learn how to do this, go to college, get a degree in aeronautical engineering.

Seriously, there is not a simple plug in formula for this. There are so many factors in the shape of the plane that contribute to the drag of the airframe, including air temperature, humidity, and altitude (air density). Essentially, you reach top speed of the aircraft when the forces of drag equal the thrust output of the engines. Calculating the theoretical drag of the airframe is the hard part, and is usually done with fairly advanced computer simulations.

Brad