I think Edrat is wrong on getting no heroes with negative luck though.. I'm pretty sure I've seen them while I take my usual O3M2 stats.
I've heard pplz say that base chance of getting a hero is 3% per turn and each luck scale + or - adds or substracts 1% point from that base.
I have to add though that in my current SP game I'm not getting the heroes I should've gotten according to that calc though

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