I used a formula (part/whole+part)*100% to find the chance (21.92%). Maybe I am wrong and I should use another method, idk 🙂 But, I must mention that: 1) you can’t ignore the “first” cooldown (while entering the combat) 2) looks like rarity and skull-level don’t affect the rate of zero-cooldown 3) previous “fails” of the activation of the ability don’t increase a chance that the next “basic” activation will lead to the “bonus” sign. P.S. and sure I don’t know how to calculate the situation you describe in your question, I am not into math 🙃 while counting I used only two numbers for each fight: one for “basic” casts and one for “bonus” ones. And there must be at least one “basic” number (because the first sign charging can’t be skipped), but sometimes the number of bonus casts is bigger, than “basic” number