as said above, we don't. soundstage is an interpretation of the brain, and as such it can be very subjective and doesn't necessarily relate to the sound. it's like asking to quantify love. sure enough we can all tell when bob is in love because some of the cues are obvious, and when you read a love story, you can often feel like you can relate to at least some cues and feelings. but you can also get a lot of confusing cues and totally misinterpret how people feel or will react to the same combination of cues.
for soundstage it's the same thing. we know how ILD and ITL work https://en.wikipedia.org/wiki/Sound_localization ,but you can't determine the soundstage interpretation only with 1 objective cue. it's timing, loudness, signature, and variations of all of the above between left and right ear. to add to the complexity, the shape of my skull, the distance between my ears, and the shape of my ears, are all important factors in how I will interpret the audio cues.
and of course we stop there only if we consider a person in the dark with no extra sensory cues and no preconception about the sound heard(which never happens). when I watch tv with headphones, my brain tries to put the voice of the talking dude onto his moving face. sometime that fails because there are just too many contradicting cues, but if the brain can do it, it will because brains really don't like confusion between visual and audtory cues(and BTW the brain trusts the eyes more!).
let's just say that you're part of how good the soundstage will be for you. it's not only you, obviously it's the sound too, but it's also part you. and what you feel is only that, how you specifically feel. people talking about the soundstage of a device are IMO wrongly making a transfer of their feelings onto the device. there is no certainty that someone else would agree with how they felt. they just assume that because they felt a certain way, then the device "sounds" that way.
