When to quit?

[Atropos]

Hey folks,
this question has probably already been asked but I dont know what exactly I could search for...
I play the NL 6-Max Games at Party, 4 tables at a time. The stack size there is 100 Big Blinds. My Standard Deviation (insufficient sample size) over 100 hands is 50.3 BIG BETS meaning 100.6 Big Blinds / 100 hands. What does this number tell me?
Is it possible to use this number and get a number of lost buyins that is unlikely enough, that it would be better for me to quit, since I'm a player that cant control tilt too good? I want a number like "lose 2 buyins - quit for the day", but this number may not be too small because I could not play too often [img]/images/graemlins/smile.gif[/img] and this number may not be too big because I could have been tilting for a long time already...