Reverse statistics

In the chapter on the physico-theological argument, you argued that the chance of God's existence (from the current argument) is one less than the chance of coincidence.
You also explained there why the claim that this is reverse statistics is incorrect. And indeed it is.
But I still don't understand why this calculation method is correct:
If the chance of a random occurrence is p, this does not mean that the other chances are a deliberate hand. It is possible that the other chances are in the option that this thing will not happen at all. Although if it happened, there is enormous bewilderment here if we say that it is a coincidence, and therefore it is appropriate to conclude that there is some kind of hidden hand here, but the way to calculate the chances is often less than the chance that it happened by chance.
Example of a thing:
Let's say that initially there was a fifty percent chance that the world would be as complex as it is now and a fifty percent chance that it wouldn't. Now that it's happened, the chances that it happened by chance are twice as high as they were in the beginning, since initially there was a fifty percent chance that it wouldn't happen at all.
In conclusion, I don't disagree that it's still pretty clear that there is a deliberate hand here, but I didn't understand why the probabilistic calculation written in the book is correct. (Of course, the chances that there is a deliberate hand here are one less than the chances that it happened on its own – because those are the two possibilities we are familiar with – but the chances that it happened on its own after it happened are not equal to the chances that it will happen in the future).
 
Hoping for understanding.