The only key is taste. A lot of people say that a 2:1 water to coffee ratio pulled in 28-35 seconds is what you want to aim for. That’s a great place to start, but I love some sweeter light roasts at like 40 second shots for 40g of water, 20-21g of coffee, and pretty much no one would recommend that based on averages.

So know what under and over extracted shots taste like, then aim for 40-45g water in 30 seconds with a 20-21g coffee dose, and decide if it seems more sour than you’d like (under) or bitter/cloying (over). If under, adjust to increase the brew time (experiment with both grinding finer and increasing the dose as the 2 best ways to increase extraction time). If you want to decrease the extraction time, your best best is to grind more coarsely.

So learn what tastes under-extracted to you and what tastes over, and then you’ll just have to adjust grind size and dose for each bean for the flavor you like most. Keep those settings and brew by weight until you need to adjust again.