bill1952
SAWHORSE
In my experience with historic theatres - I've worked on quite a few restorations and have toured many many more - it's rare that an upper seating level isn't well over a third. Can happen, just not usually.Based on the sketch, I think the entire balcony and its associated lobby can be considered to be a mezzanine.
As far as including the upper lobby as part of "the mezzanine" I wouldn't. An essential part of a mezzanine is it's openess to the main floor and the ability to see a hazard, like a fire. That view is the reason for so many relaxations from a second floor requirement.