It's not necessary the opponents stack size and total pot that needs to be over 8.5:1. A common mistake is that one tends to overestimate their implied odds
, always assuming that, if you hit your set, you'll always stack the other player. You probably need MUCH more that this amount, depending on the player you are against. If you know the player is an aggro laggard who'd go all in after the flop, 15x your stack is more than sufficient. Against A nit raising with AK, JJ+, and who will go all in after the flop with his overpair you'll probably 20x to account for your losses when you get set over set. Against a rock post flop, you are better of folding or 3 betting your pocket pair,you have no implied odds.
Hope this clears up a lot!