You also have to realize that After Effects only renders one frame at a time and a HD frame, when compared to 32GB of RAM isn't very big. Memory usage depends entirely on what's going on in your composition and what codec you are using. I'm not saying that 15% is always the best, but it could be for certain projects while a much higher usage may be shown for others. I have found the recommendations that Todd supplied through his link work quite well for just about every situation I've ever run into. It's highly unlikely that you can achieve 90% system resource usage in any more than a very small percentage of your projects.