Jake,
Sorry to say that it is a whole host of variables that can alter the fde, not a simple one of two. In the case of the approach speed and landing speed there is no set parameter.
For example a real aircrafts approach speed is determined by lift/weight/drag/power. the AI model is no different and changing any one of those parms can altering some other aspects of the flight. Likewise other parms can affect the approach/landings speeds.
A useful guide you can see here,
http://janswebsites.ja.funpic.de/fde/ai ... tings.html and download a pdf version if you want to. Jan describes the changes made to various parms in both the .cfg and .air files and what effect they have on the models flight characteristics.
Hopefully it is easy to follow but simply put you see what the changes were on the left hand side and it shows you how it affects the flight through a variety of conditions. Once you grasped what either the increase/decrease does and you want to apply that to your model then you apply a similar approach.
The bottom line is that it is a balancing act. You may find that you need to alter one parm to achieve what you want and tweak two others to counteract the effects that you didn't want from the first change.