Due to variables beyond the control of the engine builder, the notion that the rocker arm contact pattern on the tip of the valve is supposed to be in the center of the valve tip is not always correct. Items such as the rocker arm radius (the arc of the rocker arm tip travel), the difference in distance and angle between the rocker arm and valve stem centerlines, the valve tip height and push rod length combine to create the contact point. To be sure that you benefit from the entire lifter rise created by the camshaft lobe, you should always ensure that the rocker arm arc is at 90^o to the valve stem centerline at mid valve lift. Fine-tuning the rocker arm contact point to achieve the same point of contact at valve closed and max lift will produce the desired 90^o angle at mid lift.