Well, the best way to do this would be to use an adjustable pushrod on the base circle of the cam with the rocker arm in place. Then adjust it back until the plunger in the lifter is just at the top of the lifter (0 lash), but the pushrod is not loose in the rocker (almost loose, but not loose). Then measure the length of the adjustable pushrod against the pushrod that was originally from that lifter. The difference is the amount of preload on the lifter. You would then go on to the rest of the lifters and repeat the process. Keep track of the 0 lash #s for all of the lifters. Order pushrods that are about .030 longer then the longest (largest) 0 lash pushrod length. This way you will have the minimum preload on the longest distance and a little more on the shorter distances. Keep your current pushrods if the preload range is within the specifications in my first post.
Again, I think this is a case of dirt in your lifters. You may want to take them out, disassemble and clean them. Be sure to put the lifters and pushrods back in the same place they came from. The cam lobes, lifters and pushrods wear in as a unit.