Forms authentication timeout vs sessionState timeout
They are different things. The Forms Authentication Timeout value sets the amount of time in minutes that the authentication cookie is set to be valid, meaning, that after value number of minutes, the cookie will expire and the user will no longer be authenticated—they will be redirected to the login page automatically. The slidingExpiration=true value is basically saying that as long as the user makes a request within the timeout value, they will continue to be authenticated (more details here). If you set slidingExpiration=false the authentication cookie will expire after value number of minutes regardless of whether the user makes a request within the timeout value or not.
The SessionState timeout value sets the amount of time a Session State provider is required to hold data in memory (or whatever backing store is being used, SQL Server, OutOfProc, etc) for a particular session. For example, if you put an object in Session using the value in your example, this data will be removed after 30 minutes. The user may still be authenticated but the data in the Session may no longer be present. The Session Timeout value is always reset after every request.
The slidingExpiration=true value is basically saying that after every request made, the timer is reset and as long as the user makes a request within the timeout value, he will continue to be authenticated.
This is not correct. The authentication cookie timeout will only be reset if half the time of the timeout has passed.
See for example https://support.microsoft.com/de-ch/kb/910439/en-us or https://itworksonmymachine.wordpress.com/2008/07/17/forms-authentication-timeout-vs-session-timeout/
From what I understand they are independent of one another. By keeping the session timeout less than or equal to the authentication timeout, you can make sure any user-specific session variables are not persisted after the authentication has timed out (if that is your concern, which I think is the normal one when asking this question). Of course, you'll have to manually handle the disposal of session variables upon log-out.
Here is a decent response that may answer your question or at least point you in the right direction: