6lb but only for the final hour of the flight, for the next plus last hour 6lb + 3% = 6.18lb, the hour before that 6.18lb + 3% etc. etc. etc.
In my day some said 4% per hour, some said 6% for the first hour then 3% per hour thereafter but certainly with jets we had computerised flight planning to work all this out for us.
Harry, that's not how we applied it. Just 3% x wt diff x hrs. It was limited to a defined change of gross weight; the precise figure I don't recall, but I think it was a couple of tonnes. For a greater weight change we would get a new computer flight plan.