E = r / (p + 2r)
Wolfram Alpha: http://www.wolframalpha.com/
What I meant by overlay in the pot is when the pot is laying me better than break-even payoff against my equity, e.g., I have 20 left and 25% equity, break-even is a pot with 60 in it but the pot has 85 in it. It's the flip side of the cost/risk of my wager being less than my equity.
I agree about your multiway preflop point. That's not what I was thinking about. I was thinking about flop semi-bluff jam/shove situations or turn call all-in situations, where you are pretty sure you have an overlay. Like on the turn if I only have 10BB left and there is 190BB in the pot after Villain shoves (has me covered) and I've got at least 20% equity, I'm never folding that.
Great short, DJ.