How to calculate average velocity in JIRA v.7.3.2

[BigD]

We've recently upgraded to JIRA v 7.3.2 and could see team velocity in the velocity chart for any given sprint. However, my executive VP is asking to get the average velocity of 15 teams that can be useful for future predictability purpose.

Also looked at the control charts but those are based on "days" but not "story-points" so looking for simple ways to know the average velocity (in story-points) for the future predictability stand point.