(a) Many people will base their performance on something other than max manual
think we might be confusing certification with "routine" operational performance. For certification .. and maximum braking means just that .. the landing is somewhere between terrifying and horrifying. For routine (ie typical pilot, etc) the 1.67 factor gives you a fair bit of room with which to play in looking at the AFM. For everyday operations, we normally have a variable amount of additional fat in that the runway is longer than the AFM scheduled requirement.
(b) usually before touchdown
perhaps just after touchdown .. but nonetheless impressive if one is on the aeroplane at the time ...