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I'd say that a person doing the same motion over and over can be pretty consistent actually and the tamping folklore I've followed says you shouldn't tamp too hard (who knows), but I see the reasoning behind high compression. I'll give it a go.
neu-
You can get tampers with a built in spring so they always press at the same pressure. It can help reduce one of the variables.
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This gets really complex really fast. But suffice to say that you’ll never out tamp the machine, 9bar at the puck face is more than you can do.
That said, there is def a limiting point to which the benefit of tamping stops, and this will be different for each grind size and dose, but not that different as it has a small impact. The big issue is over tamping as this causes a high density area in the center of the puck.
Sumo
Matisse
My point was you want it as compressed as possible - anything else is not repeatable by a human making a mockery of your careful attempts to get pressure, extraction time, heat and volume consistent.
So there is no scale to tamping - it’s either tamped or it ain’t. ‘light tamp’ is a nonsense - unless you are not trying to make espresso and just pissing about. Which is what the guy was doing, it turns out.