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. Navigate to Lists > Manage Lists
2. Choose the list for which you want to see engagement
3. Click the dropdown menu next to Import Contacts
4. Click Engagement management 

Since engagement is measured by opens and clicks this is the best place to pull for lists specifically.

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

Related Resources

Engagement Management tool