The Sip Coffeehouse article on Sour Espresso gets specific: suggests 2 grams less coffee for lighter roasts (no scale so I used scant scoops) and 20-30 seconds to pull a double shot. Intelligentsia shows a picture (on right) of the ground coffee. It looks about as coarse as I use for brewed coffee. That didn’t work well, pull done in 11 seconds (6th try). I went back to my usual setting.
Somewhere way back I remember someone saying to put the portafilter on a scale and press 20 pounds with the tamper. That seemed a bit much. I have been tamping just enough to press all the grounds into the portafilter. 20 pounds is a little more than 2 gallons of water in plastic jugs. I carry that frequently. So I leaned into the tamping. Definitely better and hit 20 seconds on pulling a double shot. Good taste (7th) try. I prefer my lattes a little more coffee forward. This morning I used level scoops of coffee, usual grind, 3 seconds of pre-infusion, and 30 second wait before pulling the rest of the double shot. Good balance between coffee taste and bitterness, no sour taste.
I use the scoop that came with the espresso machine. It is maybe 15% smaller than the standard 2oz/60ml coffee scoop I use for brewed coffee.