This is a really handy tuning reference chart:
http://www.240edge.com/performance/reference.pdf
And this is a tuning calculator someone made specifically for Forza:
http://forums.forzamotorsport.net/forums/ShowThread.aspx?PostID=4859908
On an unknown-to-me car, after any weight reduction as a result of upgrades, I'll throw it in a calculator (like the one above) for a baseline, mostly for spring weights and alignment settings.
After that, I take the car for a test drive on a track similar to the end goal of the car, rip off a couple laps, then pull up the Telemetry. From there, I'm looking at tire temps, tire pressures, and especially the tire temps across the tire. In general, I'll shoot for roughly 32psi hot from all the tires. In addition, I'll adjust the camber to be fairly even across the tire (three sections are measured across the tread), usually in the ballpark of the inside edges being about 10 degrees hotter than the outsides.
From there, I know I've gotten somewhat close and I can start focusing on fine-tuning the balance. I tend to steer away from big swaybars, in favor of upping spring weights when necessary (which gives the added benefit of being able to run the car lower). Also, the first link I posted has a useful section on adjusting the shocks' compression and rebound - great for further fine tuning.
My goal is for every car to handle as well as racerfink's MR2 in the club's garage :D