Quantifying stereo image performance of a given reproduction chain (remember we need to include setup and room) would be complex, simply because here are many different elements to consider. Getting hung up on a single unit of measure is as others have said, a misunderstanding of the problem. But the test signal/s shouldn't be a problem, should they? You'd set that up like any controlled experiment. You'd likely start with recorded tones and claps, not FKA twigs.Because I thought the question was...."HOW do we measure soundstage".....??
If there is nothing real to compare it to, nor any unit of measure......
I never said I cared or not about it being real or not, but I thought we were discussing how IT can be measured was my only point.