I have tried this roll center site, which is free to use. Site is very simple to use with the diagram to show each value you need to input. There is also help (?) if you point the mouse on it, it tells you in words what values it needs. I also own professional roll center software given to me by my friend who works for Bush team. I compared my result from this site to professional software there was difference of only 1000’s of an inch. Professional software have very high learning curve and it gives to much information that you may or may not need. If you are only interested in knowing or designing roll center for your car I would highly recommend this site.
Now this site has added Wheel rate calculation also and is open for testing and suggestions.
http://rc-center.com/servlet/racing