Forms Authentication Timeout vs Session Timeout

In my asp.net website i am using asp.net form authentication with following configuration

<authentication mode="Forms">
    <forms loginUrl="~/Pages/Common/Login.aspx"
           defaultUrl="~/Pages/index.aspx"
           protection="All"
           timeout="30"
           name="MyAuthCookie"
           path="/"
           requireSSL="false"
           cookieless="UseDeviceProfile"
           enableCrossAppRedirects="false" >
    </forms>
</authentication>

I have following questions

1. What should be timeout value for session because i am using sliding expiration inside form authention due to which session will expire before form authentication. How can i protect it?

2. After formauthentication log out i would like to redirect page at logout.aspx but it is automatically redirect me at loginpage.aspx. How is it possible?