Therein lies the rub of the whole thing.
One good thing about being able to write good ads is it gives you more leeway to experiment with different calls to actions to get the ROI increasing as well.
If you don't syndicate then 0.5% is the figure you need to stay above, if it's less than that or hoevering near that then chances are you are wide of the mark on relevancy.
As much as I've tried to argue relevancy with Google about ads, generally speaking the customer is always right, and their figures are annoyingly accurate too.
However, that was in the early days of managing accounts, now we don't have a problem keeping things over 2% as a general rule. We have accounts in the 20% range for high volume search terms, but usually that is helped by a turbo lift into the top slots for certain keywords, which may only happen in certain categories.
If you can get above 5% CTR then you might find your ads getting lifted on top too, that is the figure we try to benchmark against if we can.