There is not a minimum score of 70 for featured hubs. If you check my hubs, all of them currently have featured status (except the one I just added that is pending.) Most of them are 69 or lower. Traffic has a lot to do with the hubs score as well. As traffic drops off, the score goes down.
As I understand it, the moderators check over each hub that is posted, and if they feel it is creative and or informative enough, they mark it as featured. This may help clue you in to your highest quality hubs.
Though I am still very much learning.
If you check on HubPages FAQ's the following is listed:
"What are Featured Hubs?
When our Quality Assessment Process (QAP) identifies articles of particularly good quality, these Hubs are given a special status on HubPages that we refer to as Featured Hubs. Featured Hubs enjoy the following benefits:
They appear on HubPages' Topic Pages and others' Hubs
They are presented to external search engines
They are included in your public HubPages Profile
Hubs that have been recently published or updated, but not yet assessed by the QAP, will appear as "Pending" in My Account > Statistics. After being assessed by the QAP (which typically happens within 24 hours) their Pending status will be removed. If they become Featured, they will be identified by a black "H" symbol in My Account > Statistics.
Why are only some Hubs Featured?
At HubPages, we strive to highlight and promote the great Hubs, so instead of presenting all Hubs on our site’s major pages and making them available to be shown in search engine results, we carefully analyze each Hub to determine a curated set that we will showcase.
The overall quality of Hubs that we highlight on our site and make available to casual searchers is important because we want to create the best possible experience for readers while also giving Hubbers the opportunity to hone in on their online writing skills in a safe, supportive, and informative environment."
I hope this helps!