For your 280x cards, look at
http://seemybits.org then adjust from there. I've found my best initial settings for my Gigabyte 280x and Sapphire 280x cards came from there. I recommend starting out here and tinkering from there. My settings are very stable and can sleep through the night. PS Remember to always factor in your rejection rates. Don't just look at your Kh rate, (K/h minus rejection % = true output) I get about 1% rejection rate.