the problem is: will there be a fall of Roman empire? Not always. And it could happen earlier or later, depending on casuality. France, Spain and England are quite close together, but not in the same turn. I've considered for all of them the earliest possible date (that isn't related to another people, like Gauls or Iberians) In general, the rules I've followed are the latter one (earliest date of the specific people, always followed but a couple of times), and to try to make a regular flow of births, without booms or bottlenecks, when possible.