Way to see percent of time spent in lightning vs classic?

I have installed the Lightning Adoption Tracker app which is helpful, but I don't see a way to see how much time (even approximately) people are spending in each.  

Also trying to figure out exactly what "using lightning" means- does that mean that they were using lightning at the moment the report was refreshed?  Some percentage of time? I see in the support documentation that Using Lightning means "this custom field stores the user preference value."  But I don't know what that means.