Those are my general zones as well (except that I lose the extra 'S' and just use UR, DR, etc.). When I have an upper level, add designations like UUR (up up right) and UUL (up up left).
On a unit set, I'll often reference furniture or other features in my blocking notation (ex. 'D x sink' or 'H x sofa'), as I find that's more specific for my brain.
I find I can be even more specific with musicals (and occasionally straight plays) when there is a number line.