Exactly what I was thinking OTAK, perfect.
I also like Refars idea, and I think we can actually do both. Have a "Set variable" so to speak. Round 1 ends at 1000 BC, we know that, but round 2 could be "350-450 AD", which is set by the

and decided when the Best Ball save is selected. That could work, we still have a basic target, but having a bit of flexibility is also not too bad.
This kind of flexible turn-set actually works MUCH better in standard SGs though, where each player plays 20 turns . . . or so (18 - 22, depending on a tech, wonder, whip, trade, GP pop, etc). With this format, I think we would run into a LOT of problems if each player decided his own stop-point, unless the target was very small, say a 5 turn variable (+/- 2 from an exact date).
Maybe for this 2nd game we should stick to a set schedule, and see how it goes with the modifications we have made. If we all find ourselves being +/- a couple turns of an important event, we can add a flexible feature of some kind, either by the

or a more "general" target date each player decides for himself. We actually had some of this in the first game, some stopped early in some rounds, I actually played 1 extra turn in round 4 to finish a tech).