That's a great rule of thumb for pillars, and there is usually no problem with that. In containers however, because the container holds the heat in, even in wide mouth containers, by the middle to end of the burn you can have a way to hot candle. The melt pool can be too deep, causing the outside of the container to be too hot, and in some (very) rare, but (very) real cases, the glass can get so hot it can shatter, causing a potential disaster. It's why we chandlers stress testing so much. It seems like it should be easy, but in reality, that's not necessarily true. And every variable; scent type, wax type, wick type, etc., can (sometimes drastically) change the way a candle burns.