Ok, well, I had this long drawn-out explanation about how it's pretty ridiculous that a 5th level character with a Greatsword, power attack, and leap attack can cause up to Gargantuan sized creatures to make a massive Damage save on a critical hit (it is referred to as 'Koth-ing' and/or 'Acyll-ing' in the rest of this post). Koth-ing will do between 11.8% and 36% of a Gargantuan creatures total hit points worth of damage. I was ok with 33+% of your total Hit Points causing a massive damage save, but 12%, not so much. It would be like dying because someone shot you in the ass with a b-b gun.
Then I though about it some more. What about a blanket 50% of your hp forces the save? Sounds, ok. But in practice, you'd be making those saves left and right, especially at lower levels. a 5th level fighter has an average of 30 hit points, so every time he takes more than 15 points of damage from an attack he would have to make a save. That's a pretty low threshold, especially when his fotitude bonus is only +4.
I think that the rule is fine the way it is, but maybe add a bonus to the saving throw if the total damage done, that causes the save, is less than 50% of your total hit points