Joe Carter at the "Evangel" blog has post up in which he discusses the idea of calculating the probability of God's existence (link to post). The basic approach is start with some a priori probability of God's existence and then adjust the probability based on evidence. Here's the fundamental problem with the approach: in order to adjust the probability based on the evidence, one must assign a value of significance to the evidence.
For example, "the existence of evil" is evidence. Does the existence of evil make it more or less likely that God exists? Typical atheistic responses are that this evidence makes it very unlikely that God exists.
On the other hand, there is "recognition of moral good," which also evidence. Does the recognition of moral good make it more or less likely that God exists? Typically, Christian evidentialists try to argue that this evidence is more consistent with God's existence than with all alternative theories.
The problem, of course, is that one can't really get at the appropriate weights for the evidence. It's intuitively obvious that the existence of one tiny injustice is one kind of "existence of evil," and the reign of Stalin is a much larger example of the existence of evil.
That doesn't mean that the article or the idea of using probabilities is absolutely useless. It can be a useful conversation starter. It can help you break the ice with an atheist friend. It may even serve as a launching pad to explain to your evidentialist friend (whether Christian or Atheist) the systemic problems of evidentialism.
Ultimately, all the evidence that exists is evidence of God's existence, because all things were made by Him, and without him was not anything made that was made. The existence of evil is not contrary evidence any more than the existence (so to speak) of darkness is contrary evidence to the sun.
The fact that someone even comes to the mistaken idea that there is a "problem of evil," is evidence of the fact that God has given them a sense of right wrong - the very fact that they pose a question to his existence is evidence that He exists.
So while I don't think that there is much apologetic value in the calculations - perhaps as a curiosity it may have some evangelical value to provoke thought among our atheists friends.