The ad request clickthrough rate (CTR) is the number of ad clicks divided by the number of ad requests.
Ad request CTR = Clicks / Ad requests
For example, if you received 7 clicks out of 1000 ad requests, your ad request CTR would be .7%. (7 / 1000 = 0.7%)
Note that this metric is only available under the "Custom" metric family tab. Learn more about the custom metric family.