First, you can call mid PPs for pair value, too.
Second, addressing your question, the common rule is 10-20:1.
The
probability of flopping a set is around 12%. So that gives you about 8-to-1 against.
The idea behind the 20:1 is that not always will you get paid off, and sometimes you'll hit your set and still lose. Also, sometimes the flop and sometimes even the turn are checked through in which case you have a higher probability of hitting your set.
Also, you can decide to play your pair and don't just play hit-or-fold.
So, the 20:1 may have been a good rule in the past where you were paid off a lot more often than nowadays. But it's okay vs weak opponents.
On the other hand, vs weak opponents you can probably play your PP for pair value anyway.
Third, you probably mean the 'implied pot odds' which is basically 'effective stack size' to 'raise size' odds.
Last edited by Jever; 06-12-2014 at 12:23 AM.