Measuring the effectiveness of an Email after it is opened

Many individuals seem too looked at either the numbers of emails that have been opened or the number of clicks an email has generated.  Both metrics are extremely important, however I always suggest adding another metric called the click-to-open metric.  This metric was introduced to me a while back by EmailLabs.  Since then I have seen little chatter online about this metric.  The click-to-open email metric is a ratio that divides unique clicks by unique opens.  The sole purpose of the ratio is to measure the click-ability of an email once it is opened by a customer.  For example, if I created an email program and the first email blast of the program had a total of 400 unique opens and 128 unique clicks I would have a click-to-open rate of 32%.  This means that each time a customer opens an email there is a 32% percent chance that a link will be clicked.  I find this information extremely valuable especially for forecasting any commerce or lead driven email campaigns.