As of this day in time, I'm looking at the tome in game it says:
"Combat Effect: Allows your melee attacks to do additional harm in the form of a frenzied strike. This ability can only be invoked when below 90 percent health. The effect will replenish itself when a frenzied strike is invoked in combat."
Just thought I'd let everyone know, since most posts say 75%. Will be uploading a SS of the tome asap.