What's the best way to create a report for active contacts (contacts that have opened an email)? I want to see how my active list sizes changes M/M
Oh wow, there's too many levels I want to answer this on...
Hope this helps you in thinking through a reporting structure that adds value to your business.
Everything you said here, plus...
Please think hard about whether your Open metric has any value. The email open activity is sent back to Marketo whenever a recipient has downloaded the images. This gives both false positives and false negatives, as clients that have a setting to auto-download are marked as opens, whereas people who actually do read the content but not download the images don't. Pretty widely discussed on this platform and making it a very dodgy figure to use.
... any user of Apple Mail (iOS or desktop) with Apple Mail Privacy Protection enabled will appear to download every email, regardless of whether they ever saw the message. This further pollutes the already dirty metric.