»In which finding the cross street is a story-problem

The NYC Street Finder contains more complex rules than many spells of legerdemain: "Drop the last figure of the address you are looking to find, divide by 2, and add or subtract according to the chart below. The answer is the nearest numbered cross street. (This formula does not apply to Broadway below 8th Street because of the many streets with names instead of numbers). ... For Riverside Drive, divide house number by 10 and add 72, up to 165 St."