I hate to bring up the "F" word, but someone developed a tuning calculator for Forza 3. Basically you input the relevent data, weight, tyre width, car layout (i.e. FF, FR, MR, etc.) and it tells you what to set the suspension to. Its not totally fool proof but it gave you a setup to start tweaking your tune from. You could get it as an excel spreadsheet, or an iPhone app (I bought the app as I found it easier to input it all into the iPhone while using the console).
What I'm trying to say is that someone should develop something like this for GT5, as I personally found it to be a god send. Otherwise, I really wouldn't know where to start when tuning a car's suspension settings! Before anyone says anything I wouldn't know where to start with creating this sort of calculator either!