1. DIRECT. If at all possible, I prefer to measure directly, as this is often the most accurate, easiest and quickest. For example, you are measuring a book for a box. Put the book on a squared corner of binders board and mark the height and width directly on the board.
2. COMPARATIVE. This involves two steps. For example, you want to measure the width of a spine. You could take a piece of paper and fold it around the spine, marking the width with a fold, pencil mark, or thumbnail indentation, then this can be used comparatively to transfer the measurement to a spine piece. An additional benefit of this method is repeatability. Dividers (technically called divider calipers) also measure comparatively, and they are often used in bookbinding to space sewing holes or holes. A jig (aka. template or guide) would also fall into this category, since they are a separate materials set at a desired measurement. Examples of this include using the width of a ruler to trim turn-ins after the boards are adhered to the covering material, or a corner jig to cut a precise 45 degree corner. Also, the inner and outer gauges on a board shear (which can be set comparatively or numerically) are technically fixtures, not jigs, since the hold the workpiece and don’t guide a cutting tool.
3. NUMERIC. This is often what people think of as measuring. It involves taking a comparative measure, then transferring it to numbers, then transferring the numbers back to a comparative length. This gives you three chances to make a mistake. But for large projects, or ones with multiple workers, this is often the best method. Vernier, Dial or Digital Calipers can be used comparativly or numerically, in fact if they have Statistical Process Control (SPC) they allow a third party to track and record the accuracy, possibly placing them in a different category altogether. Digital box measuring machines, such as the KASEMAKE Book Measure, also convert physical dimensions to numbers.
There is not one best way of measuring for every circumstance, but it is good to evaluate options depending on the task.