Unless you’re talking “The Lion, The Witch and the Wardrobe” or other matters “Narnia,” you don’t generally think of the jottings of C.S. Lewis as a box office smash. But according to Mercury Theatre owner Michael Cullen, “Screwtape” has become the most financially successful show in the North Side venue’s history. “It is doing phenomenal business,” Cullen said, sounding especially cheerful. “Every performance has been at or near capacity.” The show doesn’t release its box office numbers, but based on my unofficial math and some background sources, it’s taking in at least $50,000 a week. And since “Screwtape” has been running since September, that also means it’s not far short of raking in a million bucks at the Mercury box office.