Adding altitude is easy. All you have to take into account is the change in air density. Fortunately, this change is quite linear for the first few thousand meters of the atmosphere. As most of us will be flying well below 5000 meters (15000 feet) this simple equation is quite effective:

density = 1.22 * 0.9 ^ (altitude / 1000)

where:
density is in kg/m^3
altitude is in meters (feet * 0.3048)

BTW, I also have some knowledge of VisualBASIC programming (I have VisualBASIC 6.0) and a small understanding of how propeller aerodynamics, so if there's anything I can help with, please don't hesitate to ask.
My e-mail is: [email protected]