If you don’t like metal touching your stone try a paint brush hair. It measures .2mm and gives a good reference under the loupe. Readily available off a paint brush. I have only used one brush so far and I am sure there are different thicknesses on different brushes, maybe.
Peak optical comparator is great for measuring the girdle as well as checking your meets. Several on that auction website for $ 15. range. I paid like 80 new for mine years ago after being recommended by Vern Johnson.