The small divisions between printed numbers often carry most of a ruler’s practical information. Counting those marks without identifying the scale can produce a very precise-looking measurement in the wrong unit.
Begin with the object record and the visible labels. Some tools carry more than one scale, and the two edges need not represent the same division. Keep each scale separate in a sketch before trying to interpret it.
For an invented example, suppose a numbered interval is divided into ten equal spaces. There are nine internal marks, but ten spaces. Confusing marks with intervals would create the wrong subdivision even if the unit were correctly identified.
Look for the starting point too. The end of a physical object is not always identical to the zero mark of its scale. Damage, an unmarked margin or a specialised layout may matter, but do not decide which explanation applies without evidence.
When studying a museum photograph, avoid using the onscreen size as though the tool were life-size. A separate photographic scale can help interpret dimensions, while the catalogue supplies the documented measurements.
Your note should name the visible unit, the number of divisions within one numbered interval and any uncertainty about the origin. If one of those is unreadable, say so. A careful account of an incomplete scale is more useful than assigning a familiar measuring system because the marks resemble a ruler on your own desk.
Visual note: the image illustrates the subject, not a documented event.