What espresso ratio should I use?

Ratio is the relationship between dry dose and liquid weight in the cup, weighed on a scale, not eyeballed by volume. It is the fastest way to change how strong and how extracted a shot is, and it is more reliable than shot time because time is an outcome, not a setting. Pick a ratio, then use grind size to land that ratio in a sensible window of seconds.

1:2 is the default for a reason: it lands most medium roasts in a balanced place, thick enough to have texture, long enough to have gotten past the sharp early acids. Practically, that is 18 g in and 36 g out. If you own a scale and nothing else, weighing those two numbers will improve your coffee more than any accessory you can buy.

Move looser as the roast gets lighter. Light roasts are dense, less soluble and hold a lot of bright acidity, so they need more water passing through to reach sweetness. 1:2.5 (18 g to 45 g) is a good next stop and 1:3 is not extreme for a very light single origin. The cup gets more tea-like and less syrupy, which is the trade you are accepting.

Move tighter as the roast gets darker or as milk enters the picture. Dark roasts give up their solubles fast and turn bitter if you keep pulling, so 1:1.5 to 1:2 keeps them sweet. For milk drinks specifically, a shorter, more concentrated shot survives dilution better: many people pull 18 g to 30 g for a flat white and are happier for it. That is a preference, not a rule, and worth testing side by side.

The honest caveat: ratio, grind and temperature all interact, so there is no ratio that is correct in isolation. Treat 1:2 as a starting hypothesis, not an answer. Lock the ratio, adjust grind until the time is in the 25 to 30 second band, taste, and then move the ratio by half a step in one direction and taste again. Two or three shots gets you a real answer for that specific bag.