I was told (and shown some clever reading too) to use ATC reported wind. If in doubt, ask for update. Now this is in an area, where the ATC reading can be trusted.
On any runway I operate, we get have 60% more distance comapred to what is required with max braking effort. As mentioned elsewhere, the FMGS reading has a delay of about 4 seconds on the type flown. I would not worry much about 4 more extra knot of tailwind, or 7 (*).
Example:
In the morning the Vapp was 140 kt + 13 tail wind + some temp / compressibility effect: GS = 158 @ 65 tonnes. This is kinetic energy 405,7 MJ, all perfectly by the book as per AFM appendix. Now in the afternoon, sister ship with same brakes comes in, but only limited to AFM -10 kt tail, at 54 tonnes. Vapp 128 kt + 13 tail + TAS effect = GS 146 Kinetic energy = 288 MJ. And this is illegal.
(*) DO NOT FOUL UP THE LANDING!!! Good touchdown needs to be in TDZ, and if on the far edge, max braking is prudent immediately to retain at least some margin from those 60% initially planned. Little Rock, Toronto, Jamaica, Lyon and others did not happen for tailwind, but for touching too deep.
Calculating LDG performance for every landing, adding 15% margin on top of the figures will not prevent an overrun. Going around if ground contact not positively achieved in the TDZ would! (**)
regards,
FD.
(**) that why we never practice this in SIM, do we?