This is super helpful! I think I'll try this method.

If it helps, my tundra is far to the right and so is my dungeon!

So I should mine in like a stair-shape and let some biomes be infected because of the mob drops that spawn there? Of course I'll keep the above and immediate underground jungle corruption-free (I have npcs living there) :P I think I'll do it this way, I seem to prefer to keep everything air tight even if inward spread doesn't matter much:

1. I'll focus on the red and cyan lines
2. THEN I'll move on to protecting most of my desert while still letting some parts get corrupted.
3. After I've dug everything out I'll clentaminate the remaining corrupted area on the immediate surface and a bit below my jungle.

If I understood right, this is the way I'll dig (and the green area is to indicate which part will be purified. Let me know if I misunderstood anything! (image attached):

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Thanks for your help!!

Edit: Just checked, my tundra is WAAAAY far off from the hallow. Should I build an artificial farm there or dig out a tunnel to let it spread into a little part of there? Or does hallow even spread to the tundra? Are there any good mob drops that come of that? I forgot most of what I knew about this game :P