Solid baseline 7-B. There's no calc for it, but lesser cannons from the Armegeddon Machine could destroy towns, the main cannon trumps them by virtue of size alone, and the fact that Xander stated multiple times that he was going to doestroy Newhaven with the cannon.
Thing is, I was unsure myself. But Xander kept saying he was going to use the cannon to destroy Newhaven, and it was backed up by multiple characters agreeing that it would. Plus, the cannon casually destroyed a smaller city in the link on Knack's page.