All you are doing with the dial indicator is measuring how much each pushrod moves up and down. If one is different than the others, the next question is why?
Could be the lifter is stuck in its bore, maybe by varnish or other crud. In that case you should be able to see whether the affected pushrod is returning all the way back down or not. Compare to the other pushrods. If it is not, you might be able to unstick it with some sort of engine oil treatment, but if that doesn't work, you might have to go into the valley and clean up the problem the hard way.
If it is, that means the problem is that the lifter is not pushing the pushrod up enough. Since these are hydraulic roller lifters, the problem is not likely to be cam lobe wear, but something to do with the hydraulic part of the lifter, so the pushrod is not coming all the way up to actuate the rocker. Cleaner in the oil might fix this, but more likely you have to go into the engine and repair/replace the lifter.
If the pushrod motion is normal, the ticking noise must be coming from either loose valve clearance, or a lifter with a mechanical defect. You should be able to check valve clearance by pushing on the rocker arms. If one is loose, it could be because of a bad adjustment. You can fix this by rotating the rocker retaining nut. There is probably a YouTube on this, but it's real easy, just listening for the "click" then turning in a specified rotation on the nut.
If there's a mechanical defect with the lifter, say a bad roller, you 'd have to go into the engine again...
I know this is oversimplified, and I want to make sure to say that I am NOT a qualified mechanic, so for sure go see someone who knows way more than me.