A sourdough starter is flour, water and time, and the only thing you actually control is the ratio you feed it at. Doing that by weight rather than by eye turns an unpredictable jar into something that peaks when you expect it to.
Reading a feeding ratio
A ratio of 1:2:2 means one part existing starter, two parts flour, two parts water, all by weight. Twenty grams of starter, forty of flour, forty of water gives a hundred grams of fed starter.
The seed counts toward the total, which is the arithmetic people get wrong. Building 200 grams of levain at 1:2:2 needs 40 grams of seed, 80 of flour and 80 of water — not 200 grams of flour and water on top of the seed.
This calculator handles it both directions: give it the levain weight you need and it works out the seed, or give it the starter you are keeping and it works out the feed.
The ratio sets the clock
A small ratio like 1:1:1 has a lot of mature starter relative to fresh flour, so it ferments through quickly — often peaking in four to six hours at room temperature, then declining.
A large ratio like 1:5:5 or 1:10:10 gives the yeast much more food relative to their numbers, so it takes far longer to peak — eight, twelve, even twenty-four hours depending on temperature.
So the ratio is how you schedule your baking. Feed small to bake this afternoon; feed large to bake tomorrow morning. Neither is better.
Hydration and sourness
Equal flour and water is 100 percent hydration, which is the common default and gives a thick batter consistency.
A stiffer starter — say 60 or 70 percent — ferments more slowly and tends to favour the yeast over the acid bacteria, which gives a milder, sweeter flavour. A wetter starter favours acid production and tastes sharper.
Temperature does more than hydration, though. A jar at 78 degrees Fahrenheit behaves completely differently from the same jar at 68.
Building a levain versus feeding a starter
These are the same operation with different purposes. Feeding maintains the jar you keep; building a levain makes the specific quantity a recipe calls for.
The practical advantage of building a levain separately is that you keep only a small amount of starter — 20 or 30 grams — and discard almost nothing. Keeping a large jar and discarding half of it daily wastes a lot of flour over a year.
Build slightly more levain than the recipe needs, because some sticks to the jar. Ten percent extra is about right.
When to use it
Use the levain at or just past peak — when it has risen to its maximum and the dome is just starting to flatten. That is when yeast activity is highest.
The float test is popular and unreliable: a spoonful in water floats when the starter is full of gas, which happens well before and well after the actual peak.
Better is to mark the jar level when you feed and watch for the maximum. A rubber band around the jar costs nothing and tells you more than any test.