Treat it as a starting point; specifics change with the house, the school or the kitchen.
Common mistake in one line: leaving it late and paying more. Beyond the number, a short checklist prevents most of it.
That is a general note rather than a rule; check the current details before you rely on it.
Write down what you find before you decide anything; it makes the next step obvious. Knowing this about Common mistake prevents most mistakes.
Figures are general guidance; the primary source takes precedence.
This note is part of the cavitywalls notes desk, a set of short factual pages used to study how search engines read pages.