There is nothing straightforward about this, and no, there is no such formula.
> I'm not talking about calibrated probabilities at all. Just the notion of "how confident do I feel about this?"
If all you care about is vibes / feels, sure. If you actually need numerical guarantees and quantitative estimates to make your "feelings" about confidence mean something to rigorously justify decisions, you need calibration. If you aren't talking about calibration in these discussions, you are missing probably the most core technical concept that addresses these issues seriously.