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Also, below 0.05 I received almost no traffic on the active keywords [webmasterworld.com].
Below 0.05 seems to be a no-go area.
Try the Budget Optimizer; it does seem to auto-adjust for the lowest CPC that will get the ad shown if possible. My average CPC has roughly halved since the changes came in. I do allow a much higher max CPC than that to get my foot in the door and give the optimi[sz]er some leeway.