One assumption underlying most stereo recording techniques is that the speakers are 60 degrees apart. That’s what recording labels, studios, radio stations, equipment manufacturers, engineers, researchers and amateurs like us all accept as a de facto standard. It helps to have a standard we can all agree on so we are talking the same language. It’s a more or less agreed on thing which specifies one aspect of Steve's 'honest' stereo playback system.
Likewise a pair of speakers 60 degrees apart is what was used in testing the listeners to generate the imaging location data upon which the Stereo Zoom curves are based (I checked once upon a time and William’s notes that somewhere, an AES paper probably. I don’t recall it being noted specifically in the SZ usage paper PDF, but the source is referenced there I’m sure).
So the information given on the Stereo Zoom charts does assume that a ‘normal’ stereo speaker setup will be used to play the recording back. But, what is most important about the Stereo Zoom is not figuring out the precise measurement in degrees of things but in the relative position and relationships. The Stereo Zoom can help to set things up to get a nice wide and stable image that isn’t clumped in both speakers, regardless if they are setup 40 or 90 degrees apart. In my experience when it works well on speakers it to work well on headphones too.
Michael Williams expanded the Stereo Zoom to recording setups with multiple microphones played back on speakers which are not all equally spaced at 60 degrees and the same principles work. (see the MMAD- Multi Microphone Array Design cross linked PDF chart pages on his website, and the AES papers there for details)
The basic aspect of Stereo Zoom is controlling the Stereo Recording Angle, which is the recorded angle as seen from the microphone position which is reproduced between the speakers. What we call the DIN setup has an SRA of (+/- 50 degrees) 100 degrees total. That means sound sources arriving through an imaginary window in front of the microphones which is 100 degrees wide will fill the space between speakers, regardless of the speaker spacing. All the mic angle and spacing combinations in the table I posted above attempt to achieve a similar SRA of about 100 degrees. Sounds arriving through that 100 degree wide window will fill the space between the speakers. What the table lets you do is more or less keep that DIN recording angle unchanged, while repointing the mics the way you want them.