M$20$ M$20\backslash$times $2\mathrm{.}5$ plug screw gauge was checked for effective diameter using floating carriage micrometer and reading were taken as below: $($i$)$ Diameter of standard cylinder $-18\mathrm{.}0010$mm $(6)($i$)$ Micrometer reading over standard cylinder with two wires of same diameter $15\mathrm{.}6420$mm $($ii$)$ Micrometer reading over the plug screw gauge with the wire of same diameter was $15\mathrm{.}2616.$ Calculate best wire size and effective diameter for measurement.