It really depends on your newsletter and your desired outcomes.
Professionally, we rely on Unique Click-to-Open Rate at City Cast (not on Beehiiv) — where I lead digital strategy (and previously led newsletter strategy). Because our goal is to provide useful resources, events, news links, etc., to readers, we found its the best measurement of engagement in our newsletters. Beehiiv doesn't offer nearly the level of metrics that Sailthru does, so if you wanted to do something like this in Beehiiv, it'd take some work. But I also don't think it's the best metric for everyone.
If your goal is to get people to click links on things like ads, product/content recommendations, etc. Click Through Rate would be a good measurement.
For my newsletter, I also share the whole story in the NL. And that's basically all I send out, with some standard teasing of upcoming stories, promoting referrals, etc. But I also include trivia in each email. And so my click through rate is pretty good and is what I'm mostly tracking email to email right now.
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