You need one of these: http://www.slingshotvenus.com/stamp...tyPerf5.html
This precision gauge is clear, so it can by placed against an item on piece and still align the perf.
There are smaller versions too, metal ones, that have a cut-out over the perfs, instead of a printed piece of paper. Those you can align as well, while seeing the perf through the view window.
That said, the New York N.Y. precancel is a give-away... the precancels that this exist on are from Brooklyn N.Y. (also known Osage Iowa, Cleveland Ohio, Palmyra N.Y., Minneapolis Minn., Nashville Tenn) but none from NY, NY.