There is none, its impossible to calculate something like that.
Impossible? Why?
Obviously it's not possible to make such a calculation. To caluculate the probability of life arising spontaneously, one needs to know the mechanism by which it occurred. We don't know this mechanism, so the probability calculation is impossible. It's as if you see a cat on top of the roof of a 40 story building. What's the probabilty of a cat being on that roof? You can't answer that question unless you have some idea of how the cat got there. There would be one probabilty if the cat got there by jumping in the air to try to catch a bird, getting caught up in a tornado and being deposited on the roof, and quite another probabilty if the cat got there by walking through the door of the building without being noticed by anyone, entering a stairwell leading to the roof and climbing up the stairs.