Much of the debate about Taylor's Knock-Out values is that some very experienced hunters publicised experiences that run counter intuitively, such as occasions when the .577 (Irwin) and .600 (Marsh) failing to exhibit the anticipated K-O effect.
One thing to note with Taylor's Knockout values is that he used nominal rather than actual bullet diameter in some cases, which affects the calculation. Gregor Woods put forward an alternative formula in his book Rifles for Africa that builds on Taylor's theory called Relative Damage Potential.
The formula is RDP=(WxVxA)/7,000 where W is the bullet weight in grains, V is muzzle velocity in fps and A is bullet cross sectional are in square inches (square of radius multiplied by Pi). It is an interesting read.
I don't think any theoretical formula can be universal. However, I am comfortable with one, that 500gr and 5,000 ft/lbs is the benchmark for reliable stopping power on elephant, even if most of the very experienced elephant hunters concede the .416 as the exception....