Variables:
First of all see Extraction Theory.
Dose
A higher dose of coffee means that you have to work harder to properly extract.
The best starting point to the dose variable is the basket size. Modern baskets today come with a guide to how much guide you should use. Generally speaking that is a higher limit.
Higher doses are popular today, classic Italian recipe usually goes around 8-9 g to 14-15 g for a double.
The other important variable is the type of coffee itself. Since it’s easier to extract dark coffee, it’s generally ok to use a higher dose of darker coffee wrt lighter coffee, where a lower dose it’s more suitable.
Dose should be a constant as much as possible. You can change it as a way to save coffee during the dialing in process, e.g. you can add coffee to slow the brew. In general however, very small tweaks to dose are ok in this situations where you are in the “ballpark” of good, if the coffee is terrible, you shouldn’t touch the dose variable.
Ratio
Ratio, or proportion, is the ratio between the amount of ground coffee you use to prepare your puck against the amount of liquid coffee you get out. The two thing should correlate.
Ratios example for different kinds of espresso drinks:
- Ristretto: 1 : 1 - 1 : 1.5
- Traditional: 1 : 1.5 - 1 : 2.5
- Lungo: 1 : 2.5 - 1 : 4+
(Credit to James Hoffman - Understanding Espresso - Ratio)
This ratios are broadely agreed upon by the coffee community but aren’t set in stone.
Water, in the coffee making process is a solvant, it’s what is going to dissolve the soluble material. The more solvant you use, the more extraction you are gonna have, the more water you use, the more you extract.
It’s a simple way to increase or decrease your extraction, with a caveat: Strength. More water equals more extraction, but a lower strength of the coffee.
You also lose richness, body and texture.
Variation in this variable is in the order of few grams, which still have an impact. It’s a very simple and easy way to manipulate the taste of the espresso at the cost of body, strenght and mouth-feel.
Practically speaking, you want to start with a ratio of a style of coffee you like, and dialing the coffee, trying to keep the ratio pretty fixed, and use other variables, such as Grind Size, to do the dialing. However, if you have to do a small fix, like having a tiny bit of harshness or acidity, you can dial ratio with 2-4 g of more or less water and it works.
Brew Time
Brew Time isn’t a variable per sé, you don’t change it to affect outcome, you use it as a measure to see how changing other variable, e.g. Grind Size, affected your outcome. If you grind and use a Dose and Ratio that, considering pre-infusion, gets you your cup in 25 to 30 seconds you are around a good cup of coffee, and should be your starting point.
If you start to use particular machines, like Lever, or start to use Pressure Profiles where the Pressure changes over time in a particular fashion, then you can, and probably should, break this loose rule.