For what it’s worth, I’ve seen CTR (click through rate) defined both as clicks over open, and as clicks over received. And I think the labelling has evolved slightly over time here at Klaviyo.
Just to confirm here, Klaviyo’s glossary defines these as:
Click rate: the percentage of people who clicked a link in your message out of the people received it.
Click through rate: the percentage of people who clicked a link in your message out of the people who opened it.
...
Not to further confuse it with:
Unique click rates: the number of individuals clicking a link in your email or SMS divided by the number of recipients. 😱
I don’t see it i the Custom Analytics either, but if you had to create it yourself, I guess you can add a column in your spreadsheet of the export and calculate it accordingly (Unique Clicks / Unique Opens).