Ductile iron rockers have a larger contact area across the valve tip , roller rockers have a smaller contact area. Thus no need for correction
With the varying fulcrum lengths of different roller rockers some put the contact area on the edge of of the valve during the lift cycle, you don't want this as it can wear out the valve guides.
Optimum is to have have the roller provide a .030 sweep in the center of the valve during a cycle.
The correction kit is custom made for a given roller rocker arm system and head, each application is specific.
