You need to check visually for a straight line sighted from the lifter up through the adjusting screw on the rocker @55 percent of valve lift. The pushrod should be in a straight line with the lifter at that point in travel. The rocker being centered on the valve tip shows that your rocker geometry is good which has nothing to do with the push rod length on an LA motor. The adjustable pushrod is for measuring pushrods that will be the correct length to accomplish a straight line @55 percent of lift with the rocker making contact with the valve tip in the center of the valve which you already have.
