Ultimately, the most important metric in terms of measuring predictability is the reader's own reaction: Were you, personally, taken by surprise by the directions taken within the story? At the same time, it should be noted that a certain measure of predictability is not necessarily a bad thing. Sometimes, predictability can lend an element of dramatic tension, as readers can tell where the story is going even as the characters remain unaware. (Speaking specifically concerning Poe, a classic example of this effect can be found in "The Cask of Amontillado," where Montresor practically telegraphs the story's ending in the opening paragraph: we know that he murdered Fortunato, and it is in this knowledge that the story builds its suspense. We don't know how he gets his revenge and can only watch his plot unfold.)
In any case, while "The Masque of the Red Death" may not be telegraphed in the same way "The Cask of Amontillado" is, its end is somewhat predictable, given the dramatic demands involved in the story's setup: a group of nobles retreat from the reality of a dreadful plague, escaping into decadent festivities. You have to expect, at some point, for the plague to invade their revels—it is practically demanded by the story's setup and gothic tone. Anything else would be dramatically unsatisfying.
On the other hand, however, I don't see how "Young Goodman Brown" can be labeled predictable at all (unless one has already read the story at some prior time). In general terms, it could be argued that readers should expect the satanic figure to lead Goodman Brown into disaster (and this is certainly true), but this does not mean readers should have expected to be led to the frightening fever dream that is the Sabbath, nor should they have predicted the ambiguity of the story's ending, by which this entire encounter may or may not have been a dream. Beyond everything else, however, there is the story's resolution to consider as well: In many stories involving these kinds of encounters with evil, the protagonist emerges having grown from this experience, with a newfound knowledge and awareness of that evil. "Young Goodman Brown," however, can be understood as a subversion of this same theme. Goodman Brown comes away with knowledge on the nature of evil, yes, but he does not grow from it. Quite on the contrary, he is just as black-and-white in his thinking as he'd been in the story's beginning and spends the rest of his life trapped in misery due to these revelations.