How to calculate average strain over element set?

Hi everyone,

I'm facing the titled issue, that is well explained in this formula I'm reporting below:

I tried using HyperView, asking for Strain(t) in contour panel and "Absolute Max Principal" as result type. How can I go further? I can't think about a way to do this weighted average over element volumes since I don't have volume information in output. Do you have any idea?