Man has the unfortunate attitude that he can put the whole world into neat simple numbers and formulas.
Things like shooting are far too complex for that.The 'energy' formulas count velocity too much [V squared]. A momentum formula is much better to me. I searched every bit of info for the last 40 years and before .Today we have some fine expanding bullets and in my experience bullet construction is a major aspect of performance .I carry and hunt with premium bullets.
Against that we now have more and more use of drugs which can greatly alter the nervous system. The nervous system is what is involved with 'stopping power'.
So 9mm or better with premium bullets is the way to go.
So give up the idea that there is a formula that works and never assume that you'll get your shots where you want .The rule is to shoot and continue to shoot until the perp is no longer a threat !