Q. Difficult to Evaluate percentage response?

For estimate effectiveness some response is desired from the customers. However because total numbers of readers/ viewers of media isn't exactly know so % of response generated because of advertisement can't be generated because of advertisement can't be measured. If ad is given merely through mail then this % can be calculated easily as total numbers of persons who are contacted through mail are known however if ad is given by any other source like radio, T.V, magazine then the % of viewers can't be precisely obtained as total number of reader/ viewer is unknown to us.