Sales email open rate in custom report builder

We would love to be able to report on the open rate of a sales email template within the custom report builder, like you can in a sequence’s performance tab.

At the moment, we can view the number of emails opened per sales email, but not the opened rate itself. I know we can see this on a sequence’s performance dashboard, but as we only want to view the open rate for a specific segment of contacts who have been enrolled in the sequence, we need to use the custom report builder so that we can apply filters based on list membership.

I want to see a report that shows the open rates by subject line for every template in every sequence I’ve ever sent. I’ve spent hours searching for this report - which should be the most obvious report that every marketer on the planet would want to see. Any tips for finding this elusive yet obvious report?