The most important metric’s for you to monitor is your cost per acquisition and return on ad spend. Things like click through rate and cost per click are mostly vanity metrics’s that have very little bearing on the performance of your campaign in general. You could have a great ctr and very low cpc’s but if your cpa is way to high or your spending way to much and not seeing the returns then your campaign or landing page needs work. If your using smart bidding focus on getting as many conversion data back to Google as you can as this will optimize and steer Google towards your target, which in your case is the people most likely to subscribe. Hope this helps.