Oh? By what model? In order to calculate a probability, you must have a particular model of spontaneous formation in mind. Please state it. Then please state how you have ruled out all other of the nearly countless possible models of spontaneous formation.
We'll wait...
Hint: Those who presume to calculate the "impossibility" of spontaneous formation haven't even *begun* to examine the depths of the issue. It's like the famous "expert" who calculated that a flight to the Moon was obviously "impossible" because a shipful of even nitroglycerin wouldn't have enough power to lift itself against gravity:
"This foolish idea of shooting at the moon is an example of the absurd length to which vicious specialization will carry scientist working in thought-tight compartments. Let us critically examine the proposal. For a projectile entirely to escape the gravitation of earth, it needs a velocity of 7 miles a second. The thermal energy of a gramme at this speed is 15,180 calories... The energy of our most violent explosive - nitroglycerine - is less than 1,500 calories per gramme. Consequently, even had the explosive nothing to carry, it has only one-tenth of the energy necessary to escape the earth... Hence the proposition appears to be basically impossible..."The math is perfect, but obviously the conclusion is flat wrong. Why? Because he overlooked other methods and chemicals which might be used for the task. Chemicals: There are many fuels with *more* energy content per weight than nitroglycerin. Methods: His argument only applies if you intend to take ALL the original fuel into space with you -- as anyone who has watched a rocket launch knows, most of the fuel is burnt off and left behind well before the rocket leaves the atmosphere; the energy of a few *tons* of fuel is imparted into the velocity of just a few *pounds* of payload, giving the payload all the energy it needs.
-- A. W. Bickerton, Professor of Physics and Chemistry at Canterbury College (Christchurch, New Zealand, 1926)
And so it is with the creationists who think they know everything there is about how life could possibly have first come together, and thus calculate the improbability of it with great precision.