For certainty the courts and case law would be the answer, but allowing a a margin below would seem reasonable to ensure not 'tripping' the camera calibrated for catching people, as per Tony L. The links may help.
I am aware that someone attended a speeding awareness course mentioned an individual doing 31mph in a 30 mph limit and getting a ticket and attending the speed awareness course as 'punishment' of £97 course fee as an alternative to a fine or record etc..