I uploaded a simple roll center calculator for a S30 in the uploads section.
I developed it because I like to set up my suspensions using the control arm angles. Fast and easy. If you need more info on what a RC is or does, there is large amount of info on the web.
There is also plenty of debate on this forum on where you should set the RC and I wish I could tell you the best place. But when suspension tuning everything is a compromise and "best" is usually a combination of things, depends on your goals, and will change from car to car.