Having a feat that only activate when you have 25% or less HP is absurd. If you get that low something went horribly wrong and 100% extra Toughness replenishment isn’t going to help at that point.
What I think needs to change is the skill should be in constant states of flux the entire way to 25% HP. Yeah, the skill won’t be the best, but at least it will be better than it currently is.
Make it a constant steady slope. If we are missing 9% HP, it should be 12% extra toughness replenishment. 25% missing HP should give us 33% Toughness replenishment. 40% missing HP should give us 53%. The more HP we lose the more toughness replenishment we get until it caps out at 100% Toughness replenishment at 25% HP left.