How can I pull an average open rate per list?

You can see a percentage for engagement per list with the Engagement Management tool under Lists here:

1. Click "Contacts" > "Lists" to navigate to the Lists overview page.
2. Choose the list for which you want to see engagement.
3. Click the dropdown menu next to Import Contacts.
4. Click "Engagement management."

This is the best place to pull for lists as engagement is measured by opens and clicks.

You will see the number of contacts in the list as well as the % engagement and list cleanup options. How to use the List Cleanup Tool in ActiveCampaign