Yep, it is mostly about conversion rates. But this metric includes retention rate, which almost every website (including ad supported ones) should optimize for.

Yep, it is mostly about conversion rates. But this metric includes retention rate, which almost every website (including ad supported ones) should optimize for. I agree with you that for websites just starting up, it can be hard. In that case before full knowledge is there, industry standard conversion rates and value of goals can be plugged in. Replace those estimates by actual values as soon as you have a good idea about rev sources, as you say.