Like Dave said, there's no perfect setting. It depends on the plug-ins in your normal workflow, the other apps you have open in your normal workflow, the codecs used for your normal footage. Adobe has no control over how 3rd party plug-ins, codecs, and other apps suck up memory so it takes a bit of experimentation. The guidelines are a good place to start. I didn't catch your OS. Anti virus software on Windows Boxes has been notorious for fouling up rendering. I haven't heard much lately, but if you're using MS I'd start there first.