Wondered this for sometime and have always erred on the side of timing ending when I see the damage from the attack of whoever is hitting land. However, the rules are unclear here. Here are some things to consider:

Does it mean when I select the last attack input to hit Ultimecia? And if that is the case, then won't when she dies depend on who is attacking as characters have different attack animation lengths? What about people who aren't running and have to wait for a potential standing animation before the final hit?

Does it mean when I hear the sound of the attack? This is better perhaps, but still obfuscated by what part of the sound of the attack one is listening to as there are parts of attack sounds prior to it landing on its target.

Does it mean when I begin to see Ultimecia's recoil death animation from the final hit begin? Based only on looking at in Twitch's VoD player, it seems to either follow the frame after the damage appears or coincide with it. One of these two (with damage being slightly less ambiguous as it seems to appear in a single frame) would probably have the least disagreement over where it lands as it takes into consideration all previous grievances mentioned and circumnavigates them by simply being slightly later and being visual based.

Just to mention while on topic, an alternative would be to shift timing completely to when the camera switches to focusing on Ultimecia, but that would create its own issues with existing runs.

Looking for clarity on this. Thanks kindly guys!

I've always treated it as "When damage numbers are visible". This prevents you splitting early in case of a miss , even though it's not really relevant to ff8 but in theory it could be a problem. I think I remember reading somewhere something about the damage numbers being at the peak of their animation (they are visible and move upwards for a bit and then go back down). But that seems a bit extreme.

So can we get that in the rules? : )