Gauge is a material property you create
The label suggests a starting range; it does not know your needles, stitch pattern, tension, finishing, or desired fabric. The useful gauge is the one produced by the exact combination you intend to use.
Cast on enough stitches to measure away from the edges. Work enough rows for the fabric to settle, then wash and dry it the same way you expect to treat the project.
Measure the center without stretching
Lay the dry swatch flat and mark a four-inch span across the center. Count partial stitches rather than rounding early. Repeat in a second spot. Do the same vertically for rows.
If the two readings disagree noticeably, the swatch may be too small or the tension inconsistent. More arithmetic will not repair uncertain input.
Respect the repeat after the raw count
Multiply finished width by stitches per inch to get a raw count. Then round to a multiple that preserves the stitch pattern. Add selvedge or edge stitches only after that repeat is satisfied.
Finally, divide the chosen count by stitches per inch to see the width it will really produce. This reverse check turns a rounded number back into a physical consequence.
Sources & further reading