When I was first learning, I was taught to use a wire but I never liked that idea because a wire is round. It lays against the girdle facet and then you have to sort of eyeball the width of the wire and project that dimension onto the girdle and make a judgement call. I bought a brass feeler guage set and combine whatever sizes are needed to make the .3 mm or whatever. It is not perfect because they are odd sizes and don’t add up to the .3 mm exactly but I felt it gave me a better reading than the round wire.