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I could supply a stable voltage reference to the ADC. Anyway, those details (max ADC voltage input and so on) are not related to the circuit above.

And why do you think I will only get an extra LSB of accuracy? Instead of converting a subset (42-64) of a 0-64V range into a subset of a 0-3V one, this circuit converts a 2-24 range into an almost full 0-3V range. Am I missing something on increased accuracy?

LE: I could replace R4 with a 30V zener diode (for extra safety).