Here are a couple other overclocking guides. One uses EVGA Precision, but it is exactly the same on Afterburner. In a way, you're lucky the GTX 660 Ti and other Kepler cards will overclock themselves and set their own voltage, so you can't really hurt them with a bad overclock.http://www.hardocp.com/article/2012/04/04/nvidia_kepler_geforce_gtx_680_overclocking_review/http://www.guru3d.com/article/palit-geforce-gtx-660-ti-jetstream-review/26
Essentially, you set the power limit to maximum, then work with the core and memory sliders to gradually increase speeds. Check out reviews for conservative overclocks on a reference model GTX 660 Ti and target those clocks. Test each time with Afterburner running in the background and keep pushing upward until you experience problems. At that point, back down on the OC, set it as a profile, and leave it at that.