Time Limit Across Multiple Devices Combined

I want to limit my son's screen time to 2hrs total per day - across all his devices.  He has accounts on multiple devices - two PCs, one phone and one tablet.  

How can I configure Norton Family to limit his total screen time across all his devices?  

Right now, I have to live with the ability for him to have 8 hours of screen time per day if I put the limit at 2hrs per device.  

The granularity of having per device limits is nice but there should also be a way to set an overall limit.  Can't think of a reason why this could not be implemented!