As we enter the flop, say we have KTo and flop is 952 . We think we have 3+3 = 6 outs = 24% raw equity on 2 streets, 12% in one.
pot is 100bb, villain bets 2/3 = 66bb. Our pot odds if calling will be 28.5%. How much do we need to make on latter steets to be break-even (we only have 24% and need 28.5%)?
Follow line with 25% implied odds (rounding 24 to 25) and look un x axis for 2/3 -> we will need approx 0.33 of the pot = 33bb.
Then we can call if we think that we will get 33bb on average on later streets.
Formula:
y <- function(x,z) x*(1/z - 2) - 1
z = implied odds (as a fraction)
x = bet (fraction of pot size)
y = amount to add to be break-even as a pot ratio.