Pre-pad performance OPT's the paper tables produced assumed 50% of head wind & 150% of tailwind for their calculations; so I was told by our performance engineer. They didn't mention gusts.
Broadening the discussion slightly, I asked, and never received an answer to what to do with gusts in calculating the allowable tailwind for landing: i.e. the 10 or 15kts limit. Gusts were included in head & cross wind calculations, but not tail. The performance engineer had no answer and over the years of asking the manufacturer it seemed it was up to the airline; who also had failed to provide an answer. Bon Chance. It seemed a grey area was preferable, until you went of the end and then it was your fault.