Suppose that someone makes a continuation bet of two-thirds of the pot. If you raise them to three times their bet size, then you need them to fold about 55 percent of the time for your bluff to be profitable.
Stuff like this is pure gold. Most people, me included, just cannot be bothered to do all this number crunching. My thought process goes along the lines of "this dude is cbetting a lot, I reckon a x2.5 raise will get enough folds for it to be good". Note the use of the term "I reckon". I'm basically pulling numbers out of my arse, because I'm too lazy to do it properly. I'm fortunate in that I'm naturally gifted when it comes to maths, so my estimates are pretty good on the grand scale of things. But I don't apply myself to maximise my mathematical edge. Having some factual nuggets like this burned into my memory, well it gives me something to work from.