Setting any types of limit is generally a bad idea. If the table is good you need to stay, thats it. Only when your play starts to dwindle should you leave, and usually at 50 BB is when it starts.

Why would you leave at 30 BB positive? If your opponents are bad they are bad, just continue to play good and you'll be alright. You also must take into consideration that a brick and mortar 2/4 game is like playing the party .5/1, swings will happen so you need to max them out.

And Tyler: 30 BB is nothing on any session. If you are quad tabling if you lose two pots per table you are down 30 BB. If you feel flustered after losing 30 BB then take a break, but don't quit for the day you are much more likely to win it back if you were to play longer.