The reasoning behind the numbers. Each guide takes one decision, works through the arithmetic, and points you at the calculator that applies it to your own figures.
A calculator answers a question you already know how to ask. Most money decisions start earlier than that: you suspect paying a little extra would help, but not how much, or you are comparing two loans and the cheaper one is not obvious.
These guides are written to close that gap. Every one is tied to a calculator on this site, and every figure in them comes from that calculator's own output — not from a rough mental estimate that might disagree with the tool when you try it.
That link matters because a guide and its calculator sit one click apart, and almost everyone who reads one tries the other. If they quoted different numbers, the site would contradict itself on two pages. So the figures in every guide are checked automatically against the calculator beside it each time the site is built, and a guide with a stale figure cannot ship.
Each guide follows the same shape: the conclusion first, then the reasoning, then a worked example with specific numbers, then the questions people ask after reading the result. Where a rule has exceptions, the guide names them and says who they apply to, rather than presenting one recommendation as if it fitted everybody.
Figures that change annually are not written into the text — they come from the same configuration that feeds the calculators, so updating a limit or a rate moves the tool and the explanation together. The methodology page shows which figures the site is using right now.