I have had CTR as high as 25% as pos 4 & 5 but anything after that tends to drop like a brick in my experience. But it was in a market where very few others knew how to write a good ad. The best way to find out is to set your position and lower it every few days and watch the response. Once the drop makes a marked difference then pick it up one and leave i run a while. It will also depend on the market or niche you are working in and how good your ad is worded. I must ad that normal ctr are nothing like 25%.... I am afraid to say..