What is the lowest altitude at which wyverns appear?

I'm starting a new (large) map, and I plan to build my palace in mid-air to better see the lands under my rule. I want to ensure that when I'm relaxing in my glass-bottomed swimming pool drinking a locally brewed artisanal mana potion I don't get attacked by marauding harpies and wyverns. What's the minimum height at which they appear/the maximum at which I can safely reside?

(I'm aware that having NPCs around will reduce the spawn-rate, but those shameless peasants don't get to live in the sky. They're not worthy.)

According to the Terraria Wikia, Wyverns spawn above 800 feet above ground in all biomes except Hallow, where they spawn at just 300 feet.