The FAA FAR121 always required the 15% if the 'dest'was below 300/1/2(WX) or the runway was wet!! not required for the alt',for dispatch requirements.
The CAA always accounted for wet since the 50/60's???..
With the advent of the JAr's 'wet' is still definitely there,with very definite explanations(3mm moisture constitutes wet)
320 fcom(02 04 10p3),and the 'new'FAA requirements cover wet distance req'(but it's not retroactive to cover a/c developed before 78)
The European situ' is covered by PJ Swatton's Aircraft Perf' theory
Cheers