@meganoth that ~40 blocks seems to work!
Grade is at 61, I'd dug to 39 earlier, so just for giggles I waited till 22:00 and started digging down from 39. No z's came. Got down to 17 around 01:30, cheated in a mining helmet. With light on but not swinging pick, Stealth showed 80+. Stood up and started mining a horizontal tunnel. No z's came by for tea.
Went up ~5AM. As I was placing ladders around height 50 a z started to break in. Nerd poled up and ran outside to scold him for coming by so early. He was the only one focused on my activities, but there were 6 others near-ish (25~50+ out?) the side & front. So chased them off as well.
Sidebar: the oversaturation seems to work quite well. At Food 92 ate a Bacon&Eggs (+36) and it felt like I got all the ~28 over 100 before it started to go down again. (similar w water btw) So that's a real winner to me, thanks guys!
So it seems nighttime mining is quite doable, once you're down 40+ blocks. Though I'd recommend digging your access when z's aren't running, and immediately heading up at the first sign they've spotted you.
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On Stealth. Current system makes a lot of rational sense. Afterall zombies aren't really supposed to have a mind to read, even if we were mind readers, right?
That said, and I don't have any good suggestions, if there were some mechanic that could indicate something about if the z's are locked on & coming? Thought of this while 40+ down in the dark. Could I even tell if they'd aggro'd to me?
I'm not conveying this well... ok, our ways of telling if z's are pissed are Sound & Sight. If I can't see them then I'm dependent on hearing them. But it feels like they can aggro from a significantly greater distance than I can hear them from.
Maybe that's it. When Crouched sound volume goes down (which is great when mining) so, effectively I actually reduce my 'spidy-sense', when what you'd expect is that you'd hear 'more', or further; you're focused on listening so more likely to catch things further off. Hope that makes some sense, heh.