Toughness damage reduction (like most modifiers) has no in game description, but it can be interpreted several different ways.
If you have 50% toughness damage reduction, does it:
- Reduce damage taken to the toughness bar only by half?
- Reduce the damage taken to health as though you had additively 50% more toughness than is displayed on the bar (e.g. at 25% current toughness, taking a hit would damage you as though you had 75%)
- Reduce the damage taken to health by the percentage reduction (e.g. at 25% current toughness, taking a hit would deal 75% of its full damage. With this 50% toughness damage reduction, the hit would deal 25%*1.5=37.5% total damage reduction to the attack.
I believe it has to be one of these 3, but if someone can confirm 100% that would be fantastic