I'm using Tectonic with its increased height option, so the world build height is 640 blocks. Also, there's options to increase world build height up to 2048 and down to -2048.

In particular, I've set Tectonic to increased height and I'm trying to set the spawn dimension to Atlantis. It's a water dimension, so there actually isn't a "safe" spawn location below about Y=350.

Thanks for considering this.