Skeptical theism states that if we cannot tell whether any of the evils in our world are gratuitous, then we cannot appeal to the existence of gratuitous evil to conclude that God does not exist. However, I can't help but think that we can.
The rules of probability tell us that that individual probabilities can be quite low, but their disjunction can be very high. For instance, there may be only a small chance that you will be involved in an automobile accident on a given day, but if you drive every day, the chances are pretty good that you will be in one on some day in your lifetime. Similarly, even if the chance that a given instance of a trillion cases of suffering is gratuitous is quite low, the chance that one of that trillion is gratuitous can be can be very high, and it only takes one instance of gratuitous evil to rule out the existence of God.
Coming from someone who is not a philosophy major, am I right in my criticism of skeptical theism or is it too naive?
The theism part of skeptical theism, at least if it's classical theism, must say that the probability that God allows suffering without having an adequate moral justification for allowing it is well-defined and zero, just as you suspect. But the skeptical part of skeptical theism, as I understand it, says that we can't properly assign any probability at all to the claim that a given case of suffering is in fact gratuitous (i.e., such that God, if God exists, has no adequate moral justification for allowing it). We can't, according to the skeptical part, because we can't presume to know the full range of justifications at God's disposal, if God exists. So we have to enter a "?" rather than a number (or range of numbers) into our calculation of the probability of the disjunction, which of course renders the calculation impossible. I don't mean to suggest that I accept the skeptical part of skeptical theism, but that's what it says, if I understand it correctly.