The rule guys refer to when they think about cheating on their wives/girlfriends.
Their wife/girlfriend is the 80%: she cooks, cleans, gives him all her loving, and has decent sex. But it's never 100%.
The girl he wants to cheat on her with is the 20%: she's very pretty, has a great body, and is perfect at sex. However, she's a slob/druggie/junky/alchy.
(Seen in Tyler Perry's "Why Did I Get Married?")
Bill: "I don't know, man. My wife's just not cutting it in the sex department, ya know?"
Joe: "20/80 rule, dude... think about it..."