Me, personally I need at least two fingers of firm stable white foam. For me it’s quick visual proof that the beer and carbonation are OK and the glass is spotlessly clean.
Whenever I see beer without foam I will assume it is flat and the glass is filthy. I was programmed that way.
It’s true that any amount of residual dish soap should kill the foam head. But this is why your lesser beers carry “foam stabilizers” which can stand up to soap. I guess I’m saying that you want two fingers of unstable white foam, to indicate that your beer hasn’t been laced with that stuff.