Your Shoulder & the Gunstock does not Know or Care what the pressure is.
What gets to the stock & shoulder is how much shot you throw & how fast you throw it.
There's the answer where pressure and recoil are concerned. There is no direct relationship between pressure and recoil. So as far as recoil that can be measured goes, the focus is only on shot charge and velocity. (Actually, on the total ejecta--which also includes powder and wad. But because neither of those weighs much in comparison to the shot, they're only a minor consideration.)
As far as what one FEELS . . . there you get into an area that can't really be measured. But blind tests conducted with British shotshells and reported by the late British shotgun guru Gough Thomas showed that a team of shooters " . . . unanimously voted that the variety giving the least sensible (felt) recoil were those that, unknown to them, had been loaded with the fastest-burning powder."
