Baker's percentage is a way of writing a recipe so it scales to any amount of dough: every ingredient is counted as a percentage of the flour, and the flour itself is always 100%. Hydration is the share of water against flour — 70% hydration means 700 g of water per kilo of flour.
The temperature of the dough after mixing decides how fast it ferments. To land on the one you want, the water is not taken at room temperature but worked out from it: subtract the flour and room temperatures and the heat the mixer adds.
A starter is fed in a ratio — 1:2:2 or 1:5:5, meaning parts of starter, flour and water. The ratio sets how many hours it takes to reach its peak, which is why the same starter behaves differently in summer and in winter.
Tins are converted by the area of the base, not by the diameter: a recipe for an 18 cm tin spread into a 22 cm one covers half as much area again, so the layer comes out noticeably thinner.
The calculators above do the arithmetic. What to do with the numbers in a real mix — how wet a dough you can actually handle, when to stop the bulk ferment — is what the courses are for.