Hendrix reportedly discovered feedback by walking away from a cranked amp. The guitar just kept sustaining on its own. What followed was years of empirical system identification: learning how body position, pickup selection, and guitar-to-amp distance affected feedback character. No transfer function, just iteration. That's a valid engineering methodology.
What??? Where did you get this idea?
Any guitarist in a 1940s big band would have a big hollowbody guitar and an amp. That combination is incredibly prone to feedback. Everyone worked to reduce feedback and avoid it. That's what I do with my hollowbody when I play with a big band. It's the first thing that happens when you turn up.
Hendrix did not "discover" feedback, and in fact he did not discover the musical uses of feedback - you can hear it in BB King records that predate Hendrix, where feedback makes his notes "sing."
What Hendrix did was turn feedback into an intentional musical creation that he treated as a melodic voice.