Lol the chance is 20% by default, no wonder why it is impossible to enchant anything. +9 -> +12 = 0,2*0,2*0,2 = 1/125 that sounds pretty accurate on the amount of stigmas people need to enchant one stigma to +12 (some people needed to use like 150~200 stones) With their event it should be 0.35^3 = about 1/24 which makes it far more reasonable.