The differences between the rank I, rank II and rank III of Withstand is that the rank I of Withstand absorbs 825 points of damage, rank II of Withstand absorbs 970 points of damage and the rank III of Withstand absorbs 1050 points of damage.
Other then that the Withstand tome from rank I to III still allows you to take 2 single hits to Withstand two single melee hits alone, the information about the damage migration of Withstand comes from Hulkling for I give him credit for that information.