n = (av x p)x0.67/r

n - number of chips need to be in the money

av - average stack

p - number of player currently remaining in the tournament

r - no. of place being paid.

 

Basically it calculates the average stack at the time of in the money, then says you need two-thirds of that to guarantee you are in the money.

 

For example

We are at the beginning of a pokerschool freeroll tourney. Hence, there are 10k players and average stack is 1.5k.  Let's say we wanted to place atleast above 625th position, how many chips do you need:

n = (10000*1500)*0.67/625 = 16080chips

 

So, when I am at $16080 chips, I stop playing until I reach my goal of 625th place.