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If 1 trillion diff corresponds to $42k price for BTC, what BTC price of $110 corresponds to which diff?
That should give us the end-point of difficulty leveling off and should also tell us roughly when to expect that to finish levelling.
≃2.6 billion diff = ways to go.
That'll take about 14 difficulty adjustments at 30% each. I think we'll reach that by end of this year.
But I doubt it's a reasonable estimate as it would represent about 50x the current hashing power, or add another 27000+ TH.
That's a lot of hardware, even at 400GH/s per unit (≃67K units).