Hey @bluetulip,
just checking in to share my experiences with your add-on (and encourage other maybe-a-bit-sceptics like me) to just try it out. I have come around to your add-on and I kind of love it! 
I was a bit reluctant at first having used the old plugin to track cards studied.
I think what kept me from trying your plugin was that I didn’t really understand the metric intuitively. You explain it well and I think I know what it tracks but it’s hard to grasp what that means for my day-to-day studying. However, once I installed it and used it it feels very natural. I prefer your metric to cards/minutes studied now(*). Thanks for the add-on! 
(* Beeminder users just need to know this is not really a typical Beeminder odometer and even with a flat road you will derail after a while; but the discussion in this thread helps make users aware of that.)