99% of the time I build my atmosphere from the ground up, so to speak. I don't even start with any of the presets anymore, and I think it's best that way. It's the best way to learn how to use all of the atmosphere toolos, for sure, and when you really dig into them, especially the origin points of the clouds, fog/haze heights, falloffs and settings, light, etc., you begin to really know your way around the atmosphere editor so much better. Making drastic changes gets alot quicker, too.