Folks, as it seems that wrapping the brain around the R+W>N concept is a big hurdle for a lot of users, I made a simple web page that allows you to try out the different parameters and see how they affect the system.
http://www.ecyrd.com/cassandracalculator/ Let me know if you have any suggestions to improve the wording, or you spot a bug. Trying to go for simplicity and clarity over absolute correctness here, as this is meant to help newbies. (App is completely self-contained HTML and Javascript.) /Janne
