Stonjourner spread is enough speed to outspeed the Standard Mandibuzz that is running speed for Modest Aegislash and the rest is in HP just to minimize possible body press damage from ferro, it will never do more than 50%
252+ Def Ferrothorn Body Press vs. 72 HP / 0 Def Stonjourner: 150-178 (41.7 - 49.5%) -- guaranteed 3HKO