I agree with Vauxhall.
The only events requiring any sort of serious probability calculation are those that have not happened.
Then I'd like you to give me a probability calculation for something that has not happened.
Since life evolved/created/whatever on Earth, then it has occurred. So you can't claim that life in the universe has never happened.
Right now we know of one single instance of life existing in the universe.
I say forget the drake equation. Just go on this:
Earth exists. Earth has life. Earth has had life nearly extinct several times, so life is quite stubborn. Therefore, if there exists a planet in the universe similar enough to Earth to exist in any state that our Earth has existed in during the time life has been on the planet, then we know that some form of life could exist there. We don't know it does, but we know that, based on Earth, some form of life could survive.
We also know that, given identical conditions to Earth at the time life first formed, life should form elsewhere. Physics and chemistry all that.
So, what are the odds that there is 1 planet in the universe similar to Earth enough to support the process that created life here?