I own an assortment of measuring tools, but most are metal and not getting them near my watches! Saw this and had to buy it - plastic caliper - for $2.99! It is unapologetically cheap, but will easily get you within a half millimeter, which is all you need for most stuff.
While it is easy to measure lug width, this makes it perfect. But this will also let you easily measure case thickness, straps, and other dimensions with zero risk of scratching or scraping. Way nicer than fumbling with a ruler.