One of the worst vanity metrics with email marketing is the open rate.
It's useful over a long period of a dozen emails to have an idea about how your audience uses your emails.
But looking at a single email's open rate or comparing it to another business is going to mess with you and confuse you.
The best metric are sales directly attributable to a specific email but attribution is tricky and a sale is only the last step in your customer's behavior.
An alternative metric I heard about is the unique open rate over a specific time. In other words, what percentage of your subscribers have opened at least one email in the last ... days.
For example, I send daily emails. Instead of measuring the open rate of each one, I'd want to measure how many subscribers opened at least one in a week.
If I sent weekly, I might measure unique opens by the month.
This removes some of a problems with a single open rate and since it's looking at unique opens, it'll give you a clearer picture of how active and engaged your subscribers are.
When I ran this for my own email list, it was double what the regular singular open rate was which matches anecdotal data I've seen.
Some email services make this difficult to collect though. With mine I had to run a few exports and do some spreadsheet magic. It only took ten minutes or so but that's more difficult than just seeing it in some built-in report.
If you haven't signed up for Repeat Customer Insights, it's an easy process. Just give it your Shopify store url and it'll walk you through the setup, import, and analysis process automatically. No code needed.
P.S. You could also use this for segmentation. e.g. subscribers who opened once VS 10 times or more.
Use cohorts to find out who the best customers are in your Shopify store
Repeat Customer Insights will automatically group your customers into cohorts based on when they first purchased. This will let you see how the date customers bought would impact their behavior.