In addition, when chi is at the extortionate level of 3.5, then when phi is maxxed out, the odds of player X cooperating if player Y cooperated last turn are 66.6% or 50%, depending on whether player X also cooperated on the previous turn. Thus, in order to give player Y enough of an incentive to want to cooperate - providing odds of at least 50% of ending up with CC - player X may have their own incentive to set chi to something below 3.5. As it happens, there are two mathematical constants a little below that that might be chosen - the inverse Fibonacci constant, also called psi, about 3.36, or the more widely-known pi, 3.14.