How to determine Minimal Detectable Effect for AB Test with Click-through Rate as success metric?

I am conducting an AB test on my site. I want to decide what is considered a good MDE (Minimal Detectable Effect) when it comes to click-through rate. I can understand you can choose MDE when your main success metric is revenue since you can just look at the cost of the experiment and calculate what the minimal change is to see an increase in ROI. Not sure how to determine this for click-through rate though. Anyone have experience with this?