Ruler length

A 15 cm ruler works for a 15 cm reading and fails badly for anything longer, which is exactly the problem: you do not know the number before you take it. A 20 or 30 cm ruler removes the risk entirely and costs nothing to use.

Why the short ruler is a problem

Two separate issues stack against a ruler that is only just long enough. First, if the reading turns out to be close to or past the ruler's own length, there is no scale left to read against, and you either misread the tip position or reach for a different instrument mid-session, which changes conditions between readings. Second, rulers are often least reliable right at their own end - moulding seams, a rounded corner, or a scale that stops a few millimetres before the physical edge, which is the same zero-offset problem in reverse, showing up at the far end instead of the near one. Reading a length that lands right at the last printed mark asks the instrument to do its least accurate work exactly where you need it most.

The margin is cheap

A 20 cm ruler has a comfortable buffer for the great majority of readings, and a 30 cm ruler has one for essentially everyone, at no cost beyond a slightly longer object to store. There is no accuracy penalty to a longer ruler - the marks are the same size, the material can be exactly as stiff, and what the ruler is made of barely matters anyway. The only reason to prefer a shorter ruler is portability, which is not a consideration that applies here.

What to check once you have one

A 20 or 30 cm ruler still needs a scale that runs to the physical end and markings you can read without squinting; length alone does not fix either of those. Buy for the margin, then check the same details that matter on any ruler.

Storage is where a longer ruler pays off twice

A 30 cm ruler is not meaningfully harder to store than a 15 cm one - both fit flat in a drawer - but it does mean you are not tempted to reach for whatever short ruler is closest when a proper reading matters. Keeping one ruler dedicated to this, long enough that you never have to think about whether it will reach, removes a small decision from a moment when you are already managing pressure, angle and landmark at once. That is a larger practical benefit than it sounds: a session where you stop halfway through to swap instruments is a session where conditions have changed between readings, which is exactly what taking three readings and using the median is meant to avoid.

This has nothing to do with what a rating measures. Rate Cock produces a score from a photograph, not a length against a scale of any size, and the same separation holds for a human judge's assessment, a photo-based tool's output, and what an image model can infer - none of them run out of ruler, because none of them are holding one.