The 60% and 15% are in the operating regulation, not in the certification requirements.
The origin of the 60% is hidden somewhere in a distant past. The 15% on wet runways for turbojets is more recent, late 60's or early 70's IIRC (Amdt. 121.9, Eff, 1/15/66).
Last edited by HazelNuts39; 22nd Jul 2013 at 12:15.