Many years ago whilst doing a study for an airworthiness group I was given the opportunity to inspect, various runway surfaces. The thing that struck me most at the time was, since many runways are surfaced with `macro-porous` surfaces to allow the fluid to run down through the top layer and concrete ones are often grooved - it is in practice almost impossible to accurately measure 3mm of fluid!
Be very wary of treating figures of wet contamination too literally.