@artch I would say we use the ticks of the shard were the creep died, but that can be an exploit, if you send your power creeps into another shard before they die. Can't use the shard they spawned on because that is even simpler to exploit. (Always make your power creeps on the fastest shard)
Mabye use the shard that the player has the most cpu in, cause that is we're that's players 'base' is? And if it's the same for 2+ shards go off the number of claimed rooms, then spawns, then creeps. Or if doing something like that is to heavy on the systeam, mabye just the lowest numberd shard out of the shards with the same cpu?
I don't know the best way to do something like that, cause I'm not very active in screeps yet.