Hi Abdelhady,
It may help to create "selected the episode" as an
activation metric. This would filter your users to only those that selected the episode. I'm not sure what your treatment is, but "episode listen" may also be an appropriate activation metric, as long as treatment does not affect the probability a user listens to the episode. Then you can create a metric for (max point / duration) and compare across variations.
I hope this helps.
Luke