You can do it from the valve stems as shown, but that incorporates any rocker arm asymmetry into the results. The video (no, I didn't watch it) should also mention that if the dial indicator quill is not parallel to the stem axis (not the bore axis or 90° to the deck) the reading is wrong. Any engine you're going to work on has stems angled to the bore.
If you can, safer to do it from the tappet. Yes, it's a PITA. The pushrod is also safe, but you need to bodge up some alignment fixture top be sure of getting exactly vertical motion. I would try a short section of tube with ID equal to PR OD and held in place with a magnet or clamp.