How is the open rate calculated?
The open rate is currently based off of the total opens divided by total emails sent. Sounds like that would be correct, and it's generally a good indicator, but it can be way off if the number of sends varies.
So for example if there were 1000 emails sent yesterday and only 10 emails sent today, then you're going to see a lot of opens today (because a lot of the people that received the email yesterday are going to open it today) even though there weren't a lot of emails sent today, so the open rate will be skewed.
I'll be fixing this at some point soon.