You determine a point of zero clearance by tightening the adjusting nut, with the feeler gauge in place, until you just can't turn it any more. That's zero clearance. That is not possible when the engine is hot.
You need to do that with your finger tips and quite simply you will burn your fingers. You compensate on a cold engine @ 70F, by closing down the hot clearance recommended with aluminum heads, .006" and on iron heads you add .002".
I would have to add that in this current atmosphere where yes might mean no, but not positively, where the video tape shows you did something but you say it wasn't you, hot absolutely does not mean hot. It means that you use a procedure cold that will result in a specific measurement hot.
So when you see with your own eyes the play at home plate and you are sitting on the third base side, the runner is clearly out, then you see the camera view from behind home and that's not what you saw, then you see the first base view and the runner is clearly safe, you need to rationalize the entire scenario. In any case, "who you gonna' believe, me, or your lying eyes?" (Groucho Marx).
Pick one.