There is a range of acceptable espresso grind settings on your grinder. If you get finer than that, the shots will taste flat and have an overpowering caramel flavor; if you grind coarser than the range, the shot will be rough tasting, and both sour and cutting.

In general, baskets are designed to dose with about 2/5 to 1/2 inch clearance after you tamp, and should never show a screw mark. It is Italian orthodoxy, reproduced in the Illy espresso book, that a shot is ruined whenever the puck expands into the shower screen.

US espresso frequently breaks this with this orthodoxy and relies on overdosed, coarse ground shots. However, in general, the better the coffee, the worse it will taste coarsely ground in an overloaded basket. The popularity of the Strada and VST 18 grma baskets for 3rd wave espresso is that it can use a grind fine enough for high grown coffees, light roasted coffees at the higher dose US cafes prefer.